NEWTON, MASS. - At first glance, Aaron Wolff's bedroom screams teen: a mound of dirty clothes, hefty history textbook, the collected Calvin and Hobbes. The full-page newspaper ad for "A Serious Man'' taped to his wall? Not so much. But Wolff's decorating quirk is understandable: The 15-year-old high school sophomore has a plum role in the new Coen brothers film, appearing as pot-smoking, Jefferson Starship-loving, "F-Troop''-obsessed Danny Gopnik.

Aaron, son of former St. Paul Chamber Orchestra artistic director Hugh Wolff, is no cheeky kid actor. Before filming "A Serious Man,'' he'd had no professional acting experience. His résumé reads like any youngster's who is capable of carrying a tune better than the next middle-schooler: Michael Darling in "Peter Pan,'' coroner and commander of the flying monkeys in "The Wizard of Oz,'' Conrad Birdie in "Bye Bye Birdie.''

But Aaron is -- in the estimation of his casting director and co-stars -- a natural. He exudes aplomb, a level of comfort in his own skin that's rare in general and even more unusual in a teenage boy. His parents call him an old soul. Still, staring down the camera for the first time, under the watch of elite Hollywood filmmakers, his only real qualification for the job being having made it through his bar mitzvah (he chanted his Torah portion in the movie), was nerve-racking. For about a minute.

"It was definitely intimidating at first, because you don't know if what you're doing is right,'' Aaron says of his two months filming "A Serious Man.'' "But the Coen brothers are really gentle people, actually, and really personable. You'll try something one way, and they'll say, 'Hey, why don't you try it this other way?' And then they'll say, 'Yeah, I really like that.' I had a really good connection with Joel. He was sort of like a father.''

Aaron's father, Hugh, directs orchestras at the New England Conservatory of Music. His mother, Judy Kogan, is a harpist and writer, and Aaron, the youngest of three sons, is an award-winning cellist. Early last year, the family was living in Minneapolis -- Hugh was globe-trotting as a guest conductor -- and Judy stumbled across an ad in the Sunday newspaper for an open casting call.

"I said, 'So, Aaron. There's this movie being made by the Coen brothers, and they're looking for a kid,'" Kogan recalled. "And he said to me, 'Who are the Coen brothers?'''

Aaron wasn't especially interested, but when he found out that many of his friends were going (along with almost every other Jewish boy between the ages of 11 and 14 in the Twin Cities), he decided to go along for the ride. Six hundred showed up at the Jewish Community Center in St. Louis Park, where Joel and Ethan Coen grew up. Casting director Rachel Tenner called 200 back to read on-camera, and then whittled the field down to 20 to meet the filmmakers.

"They loved Aaron from the get-go,'' Tenner says. "He's very funny and completely unflappable, but what's really amazing about him is he has this innate confidence. He's just effortless. A lot of kid actors become very rehearsed and precocious, and that's just not the Coens' world. Finding Aaron was like hitting the jackpot.''

'I almost backed out'

If landing the role was a lark, reality soon set in. The timing couldn't have been worse, with the family slated to move to Massachusetts at the end of the summer and filming scheduled to begin in early fall. Aaron would have to miss the first term at a new school in a new town, and he was starting to worry about how the movie would change his life.

"The idea of going from nobody to somebody without anywhere in between, and seeing your face blown up on a big screen, and coming to a new school as this film kid, it was sort of scary,'' he says. "The good thing is people will know you and think you're really cool. The bad thing is people will only know you as being in the movie and not want to get to know you as a person. I almost backed out at one point.''

In fact, Aaron did have a rough start when he arrived last winter as a ninth-grader at a big school where the freshmen had already formed their social circles and he didn't know a soul. It took a few months to find his footing; in the spring, he won the title role in the dark comedy "Women and Wallace.'' Anya Whelan-Smith, a senior directing the play, had no idea who Aaron was when he auditioned for the part.

"I'd had other people in mind for the role, but when he walked onstage, it was, 'Wow.' He was really relaxed, but in a way that held your attention,'' says Whelan-Smith. "He never bragged about being in a Coen brothers film. When kids found out, it was just like a fun fact about Aaron.''

Aaron confesses that being in the student-directed one-act was "actually a more incredible experience than being in 'A Serious Man.' It was a lot more real than being in the movie. I hope to audition for the musical this year. It's called 'Sweet Charity.'''

Meanwhile, Aaron is hanging with friends, mastering math and chemistry, exploring photography, even as he distinguishes himself as something else entirely. Saturdays are spent at NEC preparatory school, where he plays cello in the program's top-ranked Youth Philharmonic Orchestra. Last spring, he appeared on "From the Top,'' the public radio and television program that showcases exceptional young musicians. By all accounts, a career in music is his for the taking.

Now, with the slightly surreal prospect of a future in film upon him, Aaron sums up his outlook: "I have no idea what to expect about anything.''

His father says that whenever thoughts of agents and managers and the Hollywood machine cross his mind, he pushes them away.

"This was a once-in-a-lifetime opportunity, something to tell the grandchildren about,'' Hugh Wolff said. But if other offers of film work begin to come in, "then we go very, very slowly. You don't sacrifice high school. You don't sacrifice friends. You don't sacrifice growing up. That's nonnegotiable.''