BEFORE "Jersey Boys" had its tryout run in 2004 in La Jolla, the two central real-life characters on which it's based -- the diminutive singer Frankie Valli and the keyboard-playing songwriter Bob Gaudio -- feared that was not the right locale to unveil a musical about the 4 Seasons.

"This was not even close to being 'our audience,' or what we perceived to be our audience," recalled Gaudio, "the non-blue collar La Jolla -- Beach Boys territory, you know, and surfers. It would be the last place we would pick in the world to premiere a 4 Seasons story."

They were relieved, naturally, when the audiences stood and cheered the young actors playing them in younger days, and when theater staffers started telling stories about addicted fans, like the man who came three straight nights -- with three different women on his arm.

But by the time the show gravitated to Broadway the next year, the "jukebox musical" craze fueled by "Mamma Mia!" had dimmed with the failure of "All Shook Up," based on Elvis' music; "Lennon," which used the ex-Beatle's songs; and "Good Vibrations," which tapped the songbook of their old West Coast rivals, the Beach Boys.

True, "Jersey Boys" had something most of those didn't, a compelling story -- the tale of Newark homeboys who battled criminal records, the looming mob and their own clashing personalities to turn out hit after hit -- but there was lingering uncertainty up to opening night, recalled comedian Stewie Stone, who went out to dinner then with Valli and another old singing idol, Frankie Avalon. When they tried to reassure Valli that the show would be a smash, he responded, "You never know," and said it wouldn't make much difference to him anyway.

"No matter what happens," the 4 Seasons singer noted, "I'll still be on the road on Monday singing 'Sherry.' "

Well, "Jersey Boys" continues to sell out on Broadway 2 1/2 years later and there are companies doing it in Chicago and London, and there's a touring version getting standing Os "here, there and everywhere," as Gaudio puts it. What's more, this weekend the show is opening at the new, custom-made Jersey Boys Theatre here in Vegas, in the just-opened all-suite Palazzo hotel. A Toronto company comes next. Then Australia. Then . . . .

There are so many incarnations of the show, or new ones in the works, they have a "Frankie School" in New York to teach all the lead actors how to sing in the falsetto that anchored such chart-toppers as "Walk Like a Man," "Big Girls Don't Cry" and "December, 1963 (Oh, What a Night)."

Each night "Jersey Boys" plays in one time zone or another, it ends by informing the audience what became of the original 4 Seasons: how bass player Nick Massi, always the afterthought (the "Ringo") is dead now . . . and how bad boy Tommy DeVito lives here in Vegas, supposedly exiled to its golf courses by the Mafia . . . and how their boy genius songwriter Gaudio gravitated to Nashville, passing days now on his yacht . . . and how Frankie Valli alone is still singing, "like that bunny on TV with the battery," the show tells us. "Chasing the music."

Of course, that's a stage version of things, so a little theatrical license is expected. But there's one big factor those sum-ups don't take into account -- what the show itself, and its success, has done to their lives.

You think Gaudio really is lounging on some lake with new "Jersey Boys" to launch around the world? "I've got a boat," he says, "that I haven't seen in two years." You think DeVito wields only a putter these days? Approaching his 80th birthday, he's back in the recording studio with his guitar in hand.

Frankie? OK, true to his word, he's still singing "Sherry."

But he says, "I wish I could turn the worry button off."

Frankie Valli

WEEKS before "Jersey Boys' " Vegas opening, the real Frankie Valli has packed the Westbury Music Fair's theater-in-the-round four straight nights, and this is Long Island, it ain't La Jolla -- there's no question it's 4 Seasons territory. More than a few men come in leather jackets or high school letter jackets, even if some have canes too now, while some of the women have teenage granddaughters in tow.

But how many performers try to get audiences to sing along and nothing happens? Here, they need little prompting to sing out "I love you Baaaby!" or that chorus about how big girls, "they don't cry-i-i," and then Frankie invites them to start another song. He doesn't even say what it is -- he just has the band launch into the opening notes of "Let's Hang On" and hundreds out there sing in unison, "There ain't no good in our goodbye-in' / True love takes a lot of tryin' / Oh, I'm cryin' . . . " And when the show's over, he plunges into the crowd like a politician on the stump, shaking hands and giving one woman a rose before sauntering up the aisle to his dressing room.

There was a period when Valli resisted doing the old hits. He dismissed some as "bubble gum" and wished he could do mostly ballads, both the ones he recorded back when and others by Cole Porter, say, or Irving Berlin -- to do more Sinatra-like crooning, in other words. But he's come to accept that it's a high calling to transport audiences back to their youths, to set off memories of where they were, and who they were, when they first heard your songs. Indeed, he has disdain now for performers who won't do their hits. "It's almost like telling the audience they didn't know what they were doing," he says, "when they bought your records."

Valli does a bit in his act in which he recalls how, when he started as a teenager singing on street corners, all he dreamed of was making enough to buy a car and put a down payment on a house -- then he pauses and adds the punch line -- "and get a summer home in France."

He can remember when he once went to a used car lot with almost no money and said, "You must have something here you can't sell. I'll take it," and that's how he got a '51 Studebaker for 100 bucks and "drove it for four years without a problem."