LOS ANGELES -- On the ground floor of a shuttered health clinic several miles north of Hollywood, Zach Braff -- John "J.D." Dorian on NBC's hospital comedy Scrubs -- is spending an alarming amount of time deciding whether it would be funny to wear a "onesie" in a scene set in his bedroom.

As an extra mills about wearing sneakers with foot-high soles (so he can stand in for a taller actor), Braff is discovering that, funny or not, the oversized pajamas are wildly uncomfortable.

"I'm not accustomed to being interrupted," he says archly, pretending to admonish a staffer -- and poking fun at his status as the first among equals on the ensemble show. For Braff, an engaging young actor with a mop of unruly hair and a seemingly limitless supply of facial expressions, the notion of being a "star" is something new. His last job before J.D. was as a waiter at a French-Thai restaurant in Beverly Hills.

While Braff struggles with his wardrobe two floors below, his co-stars in the critically acclaimed sitcom find other ways to prepare for their scenes. Sarah Chalke, who portrays the neurotic but driven young doctor Elliot Reid, scoops up and cuddles a producer's puppy, one of several pets scampering around the thinly carpeted hallways. Nearby, Donald Faison, the jock surgeon Christopher Turk on the show, takes a superhero's pose, fists on hips and elbows jutting out, while his bathrobe falls open.

"Admit it," he calls out to anyone within hearing range. "You want to touch the boxers."

His boxers are, in fact, unquestionably impressive -- certainly more than Braff's onesies. But most impressive of all on the set of Scrubs on this late summer day is the easygoing, good-natured camaraderie of the cast and crew. They are, after all, only the people that NBC has picked to make Scrubs its next big hit, its next Thursday night sure thing, possibly the must-see heir to Seinfeld and Friends.

Pressure? What pressure? On the set of Scrubs, it's all about the unapologetic self-indulgence that makes the show's frenetic creativity possible. Or, as Turk might say, it's all about the love. As much as possible with a network staring over your shoulder, anyway.

Co-stars Faison and Braff enjoy each other so much that they roomed together over the summer in a Greenwich Village apartment while both had work in New York City during the show's hiatus: Faison, who has already appeared in Clueless and other films, had a role in a movie with Heather Locklear and Brittany Murphy; Braff starred in a Central Park performance of Shakespeare's Twelfth Night.

"We love coming to work and seeing each other every day," Faison says.

Now, inspired by a line in a forthcoming script, the actors have begun singing theme songs from old sitcoms. The theme from Scott Baio's Charles in Charge draws a particularly reverent rendering. Outside, John C. McGinley, who plays Dr. Perry Cox, J.D.'s caustic mentor, is storming around the parking lot, muttering lines to himself between takes. He is desperately attempting to avoid eye contact with a woman sitting in a wheelchair nearby so he can keep his focus.

The woman turns out to be not a Scrubs extra, but the mother of one of the show's assistant directors. She stops one senior producer to offer this bit of encouragement: "You have a lot of viewers 49 [years old] to dead. I know that's not your demographic, but it's a lot of people."

Indeed, a fair number of people are already watching the show, which got its start last season on Tuesday nights. But executives at NBC want far more people between the ages of 18 and 49 to see it. If you already have the Scrubs habit, they want you to make it a compulsion. So for this fall, they have scheduled it to air immediately after Friends, the half-hour, money-minting Thursday night juggernaut that is likely to come to an end next spring. With Jeff Zucker, the head of NBC entertainment, himself leading the charge, the network sees Scrubs, which has its season debut this week, as a potential runaway hit.

It could be the show's big shot -- or its undoing, if the pursuit for ratings were to lead Scrubs' producers to water down its flavor.

Not that any of this makes a particularly compelling reason to watch the show. Caring about the whims of network officials is like rooting for the guy who approves ATM fee increases at major banks.

No, there are far better reasons why you should watch, reasons that come into focus in a visit behind the scenes of the show. Above all, Scrubs is funny. It's funny because it is uncommonly well written. It's funny, paradoxically, because it has a heart and takes relationships seriously. Some doctors who watch say that combination allows the show to represent hospital life -- the back- biting, paperwork, furtive romances, office politics, and, at times, sheer boredom -- more honestly than any other program. But Scrubs does it while actually making people laugh out loud.

How often does that happen while you're watching network television?

Reason 1 to watch:

'Scrubs' is funny

Matt Tarses is paid to make you laugh. But as Scrubs' chief writer, it's also his job to worry.

He gets to worry about whether too many of the show's several strands of plot too closely resemble each other in consecutive episodes. He gets to brood over whether something on the show is too serious or too silly. He gets to worry whether guest appearances mandated by the network to jumpstart ratings will take time away from the show's primary characters. Do Dr. Cox's gibes against J.D. (he calls the younger man every girl's name imaginable) sound like they are somehow anti-gay instead of an over-the-top attempt to distance himself emotionally? Do the show's regular flights of fantasy -- J.D. with antlers as a deer in headlights, for example -- detract too much from telling the story?

Dressed in T-shirt and jeans, Tarses sits in a room painted blue on the ground floor of the old hospital. A pile of pennies sits on the thick carpet in one corner of the room, the remnant of a bet he won. Tarses grew up in television -- his Baltimore-born father, Jay, was a writer and producer on many well-regarded shows, such as The Bob Newhart Show, and his sister, Jamie, briefly headed the ABC Entertainment division. Tarses is confident about the quality of his show. And yet he constantly frets about maintaining it.

Take the seemingly hostile relationship between Dr. Cox and J.D. Without some vulnerability, he says, the characters would fail to mean something to viewers. But too much sweetness could corrode Scrubs' edge.

"We run the risk of being maudlin," Tarses says. "We're careful about it -- we fail about one in six times. When we walk that line, it's the best."

The risk is worth it, he says. "It's nice that the show seems to be about something."

Before any episode is assigned to someone on the writers' team, it is broken down into three story lines that are scrawled onto a whiteboard in a converted waiting room. Scrubs creator Bill Lawrence, Tarses and other writers map out the sequence for each story line, with an arc that shows how it progresses, creates tension, and resolves it before commercial breaks. There are seven main characters, with several supporting figures thrown in on a regular basis, and the writers try to play with all the possible permutations.

From a writer's standpoint, this show takes some liberties found on few other sitcoms. For starters, there's no laugh track and no studio audience, so writers have to work harder in honing each punch line. "It's liberating," says writer Neil Goldman. "You must have a joke every three lines for a laugh track. On so many other shows, your standard is earning the laugh track, instead of earning the laugh. That safety net is taken away from us."

The writers' room sifts through jokes viciously. In one that was rejected, Ted, a browbeaten hospital lawyer, is called unimpressive. He wanly retorts, "How about this?" and unzips his fly, only to retrieve a business card attesting to his rank as fourth in his class at law school. The joke was dumped because it didn't feel real enough to the writers.

Others scenarios feel all too real.

In one, seemingly avuncular hospital director Dr. Kelso (described by another character as someone who "may indeed be Satan himself") responds brusquely to J.D's request to allow a man to have surgery despite having no insurance.

"Young man, I'm curious," he says to J.D. "What did you think the end result of this conversation was going to be?"

"Pretty much this," J.D. offers. "Except I'm really invested, so I also thought I might try crying a little bit."

"Sport," Kelso replies, "if crying worked on me, my wife would have her own car by now."

When they want to, Goldman says, writers can offer a "10-percenter" -- a joke they know "only 10 percent of the audience will get, but they'll love it" -- followed by another right on its heels.

That results in some puckish little asides. When a nurse checks on one middle-aged patient's welfare, she remarks, incongruously, "I liked Bow Wow when he was L'il Bow Wow." A deadpan Dr. Cox responds: "She's right. Rappers grow up so fast."

The show also relies heavily on fantasies, often visual puns or quick punch lines to underscore an emotion.

"Sometimes a patient says something you never expect to hear," J.D. says in a voiceover, just before a woman lying in a hospital bed announces: "I'm ready to die." Suddenly a delivery man pops in the room and says, "Delivery for Mr. Dorian!", and a ton of bricks falls on him. At another point, when he feels alienated from Turk, who is busy bonding with other surgeons, J.D. imagines a ridiculously elaborate high-fiving ritual among them.

As Scrubs enters its second season, Lawrence and Tarses are reducing the show's reliance on such humor. "I feel like this show fails when the comedy is too broad, too goofy, so you don't buy it," Lawrence says.

There are, of course, a few thousand other moving parts besides the writing that make a show work -- the direction, for example, on a so-called "single camera" shoot, as opposed to the three or four cameras of the standard sitcom, involves much more care. The "single camera" shot makes for a much more natural feel for viewers. But if something goes wrong, if actors are off even a split-second in timing, it's much tougher to patch together from existing footage.

Tarses knows a little something about the perils of innovative shows. A former writer for Will Smith's hit Prince of Bel Air and Men Behaving Badly, a brief-lived NBC recycling of a British show, Tarses joined Aaron Sorkin, the much-decorated creator of West Wing, to work on SportsNight, a short-lived show about people who put on a show much like ESPN's SportsCenter.

That half-hour program, intended as a comedy with a serious center, routinely flouted convention. It too had no laugh track or studio audience. Drama was mixed extensively with the humor, and its dialogue was written with a very clear, rapid-fire pace -- one that was entirely Sorkin's own sensibility.

Despite its aspirations, the show didn't quite seem to work. Common criticisms of SportsNight were that all the characters spoke in the same glib, clever patter, and that the show periodically veered into melodrama.

Both assessments, though, reflect Sorkin's singular vision and voice, Tarses says. Lawrence, who created ABC's recent hit Spin City at the age of 26, has a similarly strong instinct for how he wants Scrubs to look and feel. (In the Scrubs' writers room, one of the worst things a joke can be is "too sitcom-y.") But Lawrence has proven willing to take inspiration from elsewhere.

The program itself is openly derivative -- its creators acknowledge its debt to earlier shows with medical settings, such as M*A*S*H in the 1970s and St. Elsewhere in the 1980s, but also to the coming-of-age series The Wonder Years. Lawrence is a self-described "television hawk," and laces popular culture throughout the show. But he conceived of it from the experiences of his close friend from college, a young doctor named John Dorris.

Sarah Chalke's Elliot was originally written to be a type-A, competitive threat to J.D. But the writers noticed that Chalke, a Canadian who had a major role on TV's Roseanne, talksveryfastwhenexcitedornervous, and is prone to malaprops. So they gave her character a similarly speedy verbal delivery, and allowed her to substitute insecurity for hostility.

"I hope [Elliot] doesn't have any social skills anytime soon," Chalke says, beaming.

Actor Judy Reyes found that dialogue for her role of Carla, an experienced Hispanic nurse who dates the younger Dr. Turk, was written to have her break into Spanish any time she was annoyed. When she first secured television roles, she says, she found herself doing a cut-rate Rosie Perez impression, her dialogue accompanied by an upraised palm and high-attitude head bobs. She didn't want to do that again on Scrubs. So, in an early episode, Lawrence and his writers whipped up a scene in which she tutored J.D. in the art of being a minority sidekick, at once sending up and putting to rest the stereotype.

He melds who he knows us to be socially with who [the characters] are," Braff says of Lawrence. "You can tell Bill a story at a bar over a beer, and three weeks later you'll see a version of it in a script."

The show is always grounded, however, in its hospital environment. Patients come in injured or sick. Not all of them can be helped, and not everyone cares that much about helping those who can. These bleak truths are not always the stuff of great comedy. But, as Tarses says M*A*S*H sometimes showed, it can also be moving at the same time.

Reason 2:

'Scrubs' is real -- sort of

John C. McGinley at rest is a human coil of energy waiting to be unleashed. For film director Oliver Stone, McGinley has played a series of diverting but soulless characters -- a frightened soldier in Platoon, a greedy broker in Wall Street, a self-impressed sports-talk-show host in Any Given Sunday. In Scrubs, he is Dr. Cox, the chief resident who cloaks his anger and passion with contempt. That tension leads to some of the sharpest exchanges on the show.

When Dr. Kelso (Ken Jenkins), the penny-pinching hospital director, squares off with Dr. Cox, "It's like Vader and Skywalker," says Faison. Dr. Cox is equally sharp with his ex-wife, Jordan (played by Lawrence's real-life wife, Christa Miller), affectionately describing her as a "wire-haired man-goblin."

Dr. Cox is also a self-professed narcissist, happy to champion his own virtues to all. After a small triumph with a patient, he turns to a nurse standing nearby: "When you speak of this, and I know you will, could I be shirtless? I think it would be even more impressive if I were shirtless."

McGinley's delivery as Cox involves a punched cadence with periodic pauses to ensure that his sarcasm is fully received by its target. He says it's a mix of the actor John Malkovich, a friend, and his own father, who had a habit of opening his mouth intermittently to pop his ears.

McGinley says he insisted that Dr. Cox transcend the "cartoon pirate" tag once hung on him by another character on the show. Five years ago last month, McGinley's son, Max, was born with severe developmental disabilities. Since then, McGinley says, he has wanted to weave a thread of sincerity, of true feeling, into all his roles -- even into someone as acerbic as Dr. Cox.

"Billy [Lawrence] knows I'm a lot of hot air, and lets me give love from time to time," McGinley says. "When he sees me and Max together, he knows all that other stuff is so much hootenanny. Billy takes three milligrams of that and sprinkles it into the script every once in awhile."

The show's relationships in general are unusually fully formed. The romance between Carla and Turk, for example, is beset by miscommunication, insecurity and pride, yet remains strong because of the compromises that they're always willing to make. The sharpened banter between Dr. Cox and his ex-wife is informed by their lingering affection for one another. Turk and J.D., best friends since med school, constantly test one another's patience and loyalty.

At the center of all of these is the relationship between Dr. Cox and J.D.: J.D. desperately wants to be Dr. Cox's protege. Dr. Cox accuses him of acting like a smitten puppy, and rewards the newly minted doctor by ritualistically mocking him. Then, when J.D.'s character is least expecting it, Dr. Cox is allowed to grace the younger man with a shower of affection -- before taking it all back.

"Newbie, don't ever be afraid to come to me with stuff like this," Dr. Cox tells J.D. "The simple fact that you actually seem to give a crap is the reason I took such an interest in you. Hell, it's why I trust you as a person."

The appreciative younger doctor offers to treat Dr. Cox to dinner, but when he suggests meeting after work, the older man bristles. "Oh, no, no, no. I thought it was a gift certificate kind of thing." He taps J.D. on the head with a rolled-up paper. "Eat dinner with you? I mean, come on, that's crazy talk."

"It's totally the Ed Asner school of vulnerability," McGinley says, invoking the actor who portrayed the gruff television boss Lou Grant on the Mary Tyler Moore Show. "If anybody asked, it never happened."

Reason 3:

NBC actually cares about 'Scrubs'

This is, on its face, a dopey reason to watch any show. Network executives answer to faceless media conglomerates (in NBC's case, a financial and manufacturing giant) that have no particular stake in any kind of show. They're agnostic about the relative merits of Fear Factor, Seinfeld or, for that matter, Pride and Prejudice, as long as it draws good ratings. They want the biggest audiences with young demographics.

Yet, critics hold the series in high regard. Tom Shales of The Washington Post called it "utterly lovable, a mega-dose of fresh air"; Howard Rosenberg of the Los Angeles Times found it "one of those distinctive comedies in which everything meshes"; Julie Salamon of The New York Times termed the show "funny and appealing."

And, to his credit, NBC entertainment chief Zucker has been Scrubs' biggest fan, finding ways to give it increased profile and more exposure to viewers. Scrubs had jumped around a bit on the network's schedule last year, then settled in part of the same hour as Fox's innovative 24, also popular among many younger adults. Scrubs averaged a 5.3 rating among adults, 18- through 49-years-old, or 13 percent of all viewers that age who were watching television at that time. It was not among the top 20 highest-rated shows.

Nevertheless, in late spring, Zucker announced that Scrubs would be promoted into the position after Friends, potentially giving the sophomore show a major boost. The network offered a "Scrubs-a-thon" over the summer, playing half a dozen episodes one night. And he has championed it at every opportunity.

"People put on a lot of comedies -- we put on a lot of comedies -- and quite often they don't measure up," Zucker says in a telephone interview. "This one is reaching for a very different bar. It deserves a shot on Thursday nights."

Zucker says he hopes for Scrubs to hold onto 70 percent or more of the audience watching Friends, one of the top-rated shows on television. He says he's appealed to Lawrence to stick to his quirky vision. But, he admits he's also reminded the Scrubs brain-trust that their show is appearing between two major hits -- Friends and Will & Grace. That placement, he says, "sends a message to the audience that this is a show of a certain quality, and it sends a message to the [entertainment] community that this is a show we believe in."

The landscape of network television, of course, is littered with quality shows that never made it, such as Buffalo Bill (a creation of Jay Tarses), Freaks and Geeks and I'll Fly Away.

And Lawrence hears Zucker loud and clear.

"I know he gets nervous when we're doing the emotional stuff," Lawrence says. "The biggest thing Jeff has given us, no matter how subtle he thinks he's being, is the message that there's a lot of pressure on us."

Every time a script is finished, the question becomes: how to make it funnier, tighter, smarter, better?

"We're really taking his pressure to heart," Lawrence says. "We wanted the worst-case scenario for us to be that we were involved in a noble failure."