He is supposed to be a movie star now, but Jack Black doesn't play that part. When he arrives at the Ritz Carlton for a round of interviews, Black looks more like Barry, the chubby, obnoxiously over-the-top record store clerk he played in "High Fidelity." He's got a scraggly beard, a Shins T-shirt, and jeans loose enough to reveal his briefs. The actor, known for his manic performances and a cult of devoted fans, digs into a Burger King bag for lunch. For dessert, he chews a piece of nicotine gum.

It's a big week for the greatest scene stealer of his generation. Instead of playing the sideman, Black is getting a true star turn in "The School of Rock," which opens Friday. He plays a grizzled guitarist who fakes his way into a substitute teaching gig at an elite elementary school. There, he hijacks the uniformed overachievers to train them to be rock 'n' rollers.

"I'm not trying to win any prizes," says Black, 34. "I'm just trying to make people laugh. A lot of people say, `Oh, Al Pacino, his best work [was] in the early days in the `Godfather' when he had subtlety. [Look] what happened to Al Pacino and the `hoo ha' and the over the top.' I love Al Pacino. I love him chewing the scenery. That devil movie with Keanu [Reeves]. He's hilarious. How much can I take of it? Please, more."

Over the last five years, Black's fans have had to fast-forward through hours of film to see their hero at work. He's played the heavily medicated emergency room attendant in 1999's "Jesus' Son" and the burned-out older brother in 2002's "Orange County" and has added a steady stream of public appearances to the mix, from a goofy performance with Lou Reed at an autism benefit to a hammed-up musical tribute to Conan O'Brien in prime time. Black is also one half of the mock-rock duo, Tenacious D, whose self-titled 2001 debut sold more than 600,000 copies, according to Nielsen SoundScan.

But Black's previous chance to star in a movie, 2001's "Shallow Hal," was considered a disappointment by some. He was asked to play the straight man in the Farrelly brothers comedy. There's no such requirement in "School of Rock," which Mike White ("Chuck & Buck" and "Orange County") wrote especially for Black, encouraging the kind of outrageous performance that's made him a rare combination: a cult idol with commercial reach."

He was funny in those other movies," says White, "but I don't think of them as Jack Black vehicles. This is kind of the first because the burden of the movie is all on him."

One large band

In "The School of Rock," Black plays Dewey Finn, the lead guitarist of a club band trying to impress a label executive. But the big audition ends with the shirtless Dewey turning an attempted stage dive into a nasty belly flop. The scout leaves, Finn limps home and, when he shows up at practice the next day, he learns the boys have kicked him out of the band.

For Finn, this presents a problem. Now, he's stuck dealing with a roomie, Ned (played by White), whose Dewey-hating girlfriend (Sarah Silverman) wants Finn to pay the rent or get out. It's at this low point that Dewey, alone one day, answers a call from the Horace Green Elementary School. It's looking for Ned, an aspiring teacher, to fill in as a substitute. Finn's about to take a message when he learns how much the gig pays. Quickly, Finn pretends to be Ned and takes the job.

When Dewey reports for class, he lets the uniformed kids know he's hung over. He tells them there will be no grades and no work. And when one of the students complains -- "my parents don't spend $15,000 for recess" -- Black offers his first lesson.

"Oh, you want to learn something!" he shouts. "Give up. Just quit. Because in this life, you can't win."

Dewey changes his tune after watching the preteens in a music instruction class. He realizes they're far better players than he and gets an idea. Why form another band outside the school when he's got all the players there? But the kids, he knows, will need help. They play Beethoven, listen to Britney, and have never heard of Black Sabbath. Dewey goes to work. He drills them with rock history, hands out copies of Rush CDs for homework, and shows them clips of Pete Townshend executing his trademark windmill.

Ask Black whether his brand of high-energy comedy can carry an entire movie, and he'll respond quietly.

"I don't know what other people can take. I just know what I like," he says. "And this one, I liked it. When I see myself in it, I say, `Good, nailed it.`"

But ask him about Yes's song "Roundabout," and he sits up straight in his chair.

"That is the greatest keyboard solo in history," Black insists. "You know what I mean?"

When he gets just a nod, Black looks back as if shocked, his eyebrows forming a V.  Suddenly, he's become Barry all over again.

"No, it's not just a great solo. It's the best solo. Good luck trying to find a better keyboard solo," Black says, waving his hand.  "I should also mention the best sax solo in rock. `Urgent,` by Foreigner."

A subversive collaboration

White's writing drew Black to the movie. The two had been neighbors in Black's native Los Angeles for a few years and shared a sensibility with "School of Rock" director Richard Linklater ("Slacker," "Dazed and Confused"). The film, they all decided, would not be a simple morality tale in which a reformed rocker decides that, indeed, he wants to drop Queen to teach the quadratic formula.

"This is not [expletive] `Kindergarten Cop,' " says Black, slipping into his Schwarzenegger voice. " `Look how stupid Arnold is with kids.' It's got subtle, subversive messages in there that are funny and cool. You don't see that, not just with kids movies, but with movies."

Instead, rock class gives these strait-laced kids a chance to rebel, to cruise around in Black's rusty van, and generally dupe the school's administration.

"Can we tell our parents?" one kid asks after Dewey informs them that his syllabus will include the Sex Pistols.

"No!" he screams.

Not only is the film soaked with retro-rock references, Linklater brought in Sonic Youth's Jim O'Rourke to teach the young actors -- who, like their fictional counterparts, are accomplished classical musicians -- how to rock out. Black sang his parts and played some of his own guitar. Linklater also had members of the Vandals and the Mooney Suzuki help with the sound.

"When I think of Jack in Tenacious D and the rock bands, I felt it was appropriate to choose bands that were rock philosophers, [to treat] rock as religion and not just rock as music," says White.

Not only is Black a natural rocker, he shows that he's comfortable with kids.

"I don't want kids for myself, but I am a kid," says Black, whose longtime girlfriend is the comedian Laura Kightlinger. "I'm an old stinky kid with a dirty mouth. A lot of my humor is very childish."

Does he really not worry about being a leading man? Will he ignore the box-office returns?

"When I see this movie, I say, `You know what, if this is my last movie, that's OK,' " says Black. "I've finally got the thing that has my essence on it. Yep, that's what I wanted to do, that's what I hoped it would be, and there it is. There's been a couple times like that but never with me in the lead."

Geoff Edgers can be reached at gedgers@globe.com

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