Much has been made of the vocal performance that Scarlett Johansson delivers as the title character in Spike Jonze's latest film, "Her," about a computer operating system named Samantha who falls for her human owner Theodore Twombly (Joaquin Phoenix).

There's another perfectly cast voice in the movie too -- a foul-mouthed alien child who appears inside a videogame played by Theodore. The actor? Jonze himself. "It might be his best role yet," jokes the helmer's longtime editor, Eric Zumbrunnen. Asked why the filmmakers didn't enlist a real child for the part, Zumbrunnen instantly responds, "We did!"

"Childlike" is a word used often by those who know Jonze best, including a tight-knit group of collaborators the filmmaker ferries from set to set, some of whom he's worked with for nearly 20 years. "He's a handful," says Johnny Knoxville, who collaborated with Jonze as an actor on MTV's "Jackass" series and movies, among them their latest hit, "Bad Grandpa." Adds Knoxville, "There's a lot of laughter and wrestling and pranking on a set. But in addition to a childlike enthusiasm for all things, he has an amazing, critical-thinking adult mind."

That dichotomy is on full display this month with two projects that couldn't be more disparate. Jonze receives a story and producer credit on "Bad Grandpa," in which Knoxville is disguised as an elderly man and pranks unsuspecting bystanders. And "Her," Jonze's fourth directorial feature, hits theaters Dec. 18. Like his previous turns in the driver's seat, it defies easy classification.

Jonze is, after all, the filmmaker who took us inside the mind of a character actor in "Being John Malkovich" and captured the most meta case of writer's block in "Adaptation" before bringing the classic children's book "Where the Wild Things Are" to life.

Jonze's latest endeavor, "Her" -- which just won awards for director and film from the National Board of Review -- is at its heart about a relationship. It just happens to be between a man and the voice of his computer. Jonze's script and direction takes what could have been a one-note premise and makes it a heartfelt love story.

"In other people's hands, this material could have been impossible," says Sue Kroll, president of worldwide marketing and international distribution at Warner Bros., which is releasing the film. "But he's a genius."

Jonze, 44, is one of the entertainment industry's most enigmatic talents.

It can be hard for some to reconcile that the person who helped launch youth-culture, male-targeted magazines Homeboy and Dirt, and once helped kidnap Brad Pitt on "Jackass," is the same artist behind the idiosyncratic critical darlings he has fashioned for the bigscreen.

"I'm not sure why that is. I just want to do things that excite me -- that make me laugh or touch me or confuse me," says Jonze, whose career spans everything from musicvideos and commercials to films, television and publishing.

Tucked away in a private booth at a restaurant in the Four Seasons in Beverly Hills, Jonze hardly looks like an edgy auteur or mischievous prankster. Casually dressed as usual, he has an unassuming air and is soft-spoken and unfailingly polite. The writer-director-producer-actor is more engaged by conversation with his interviewer than by answering questions only. As Kroll says, "He's interested in everything and everyone in the world. I'm always surprised when I meet people who are so engaged in that way -- especially in our business, because people look in a lot. And Spike is always looking out."

Little is publicly known about the prolific but press-averse Jonze. He was born Adam Spiegel; his father Arthur H. Spiegel III, ran a health-care consulting firm, and his mother, Sandy Granzow, is an author and artist. While he was famously married to filmmaker Sofia Coppola for four years until 2003, Jonze has never discussed the relationship publicly.

Despite the fact people know little about Jonze's private life -- or maybe because of it -- there is endless speculation on how personal "Her" is to him.

"Everything I've done feels really personal," Jonze says. "Even scripts that Charlie Kaufman wrote (including "Adaptation" and "Being John Malkovich") feel personal, which is why I was drawn to them. Even my musicvideos and 'Jackass,' I'm close to it all because it's all stuff I'm excited about."

Jonze, who is also well known for his musicvids for such artists as Weezer, Beastie Boys and Bjork, recalls that when he first started doing musicvideos, people were eager to label them as ironic. "I think something can be funny and sincere at the same time," he says.

It's not hard to find power players who rave about Jonze, personally and professionally.

Sony Pictures chair Amy Pascal championed Jonze's second picture "Adaptation," starring Nicolas Cage and Meryl Streep -- rather a bold move given the film's non-mainstream plot about a lovesick screenwriter who grows increasingly desperate in his attempt to adapt Susan Orlean's book "The Orchid Thief." "You gotta make some scripts you love," says Pascal, recalling the first time she met Jonze. "He came into my office and did magic tricks, and I was gobsmacked. He was smart and unassuming, and his genius creeps up on you when he's talking."