THEPERFORMANCE

In wet hair, overalls and no makeup, Olivia Thirlby scoots into the restaurant at L’Ermitage Hotel in Beverly Hills looking a little disoriented.

“This whole California thing . . . I’m warming up to it,” says the East Village native. “Especially in the summer, when it’s humid in New York and out here it’s sunny and beautiful.

“I don’t drive, but I like ‘passenging,’ that’s what I call it. It’s one of my favorite things to do. ‘Let’s go for a drive in the hills; I’ll just sit here while you do all the work.’ ”

The actress who came to prominence as “Juno’s” cheerleader friend seems easygoing but is apparently trying to take over the world, with six films due this year. This week’s “The Wackness” finds her as a privileged girl way above the teen pot dealer protagonist’s pay grade.

“She is the girl, the bikini girl,” Thirlby says of the role, “but she’s so much more than that. She’s genuine, I think, and she’s kind to him and she cares for him, and she’s nonjudgmental of the fact that he’s a sexual novice.

“People keep saying to me, ‘Stephanie’s the heartbreaker, the bad one, the antagonist.’ I disagree, and I hope it doesn’t come across that way to everyone because I understand where she’s coming from.”

Thirlby is just 21 but already has a bit of smoke in her voice. And at an age when most of her friends are in college, she’s working seemingly nonstop. But she intends to go to school when the rhythm of her career allows a pause.

“I used to say I wanted to go to law school, but I have no interest in being a lawyer. My secret dream job, which I would never do because it requires me to be so qualified: I would love to clerk for a Supreme Court justice. When I was in second grade -- my parents always joke about this -- on my report card, the teachers wrote that I was the voice of moral reason in the classroom at the ripe old age of 8,” she says with a delicious laugh. “Of course, it all went downhill from there.”

Then again, if she were in school, she would have missed “one of the craziest moments” of her life:

“One, I was at the Oscars. Two, I was on the red carpet at the Oscars. And three, someone introduced me to Daniel Day-Lewis on the red carpet at the Oscars. And four, he was like, ‘I really loved your work in “Juno.” ’ And the fifth step was me just falling over. I lost my balance and I wasn’t even wearing uncomfortable shoes. Whoaaa!”

She would also miss artistic highlights such as working with director Paul Greengrass (on “United 93”).

“You mean Mr. Brilliant McGeniuspants?” she says, laughing, then deadpans: “I mean, his name says it all. It was an incredible experience, that job. It really was. All the tears, that fear, that was 100% real.

“We filmed it in real time. So our takes would be, like, half an hour long. Our longest take was 56 minutes. Fifty-six minutes of submerging yourself in the most terrifying improvisation imaginable, that you’re in a plane crash.”

She stabs two blueberries simultaneously on separate tines of her fork as she considers the question of how she makes the teenage girls she plays in “Snow Angels,” “Juno” and “The Wackness” so varied.

“I think there’s a lot of different parts of my personality. I’m just like Lila and I’m just like Leah and I’m just like Stephanie all at the same time, even though they come off as these wildly different characters. Everyone is a blend of people. I think that teenage girls, especially, are possessed of different and sometimes conflicting qualities.

“The credit lies with the writers,” she says. “If you just play what they wrote, it’s the easiest job in the world.”

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Where you’ve seen her

For now, Olivia Thirlby is best known as the peppy best friend in “Juno” (“It’s probably just a food baby. Did you have a big lunch?”), but, with best-in-show roles this year as the earthy outsider in “Snow Angels” and the sorry-to-break-your-heart girl in “The Wackness,” she’s poised to be voted most likely to succeed. She has four more films slated for this year, with two more expected in 2009. Due in 2008: a small role in “Margaret” with Matt Damon; a turn as a snarky goth girl in “Safety Glass”; “Uncertainty” with Joseph Gordon-Levitt; and a brief role in “Love Comes Lately.”