So far, praise has focused on Thirlby’s “natural” and “authentic” teen presence and how she manages to deliver big with such small parts. In fact, ask her what she brought to Juno, and you’ll hear her moxie surface again. “I basically like to take all of the credit for the story,” she says. You can laugh, but she’s not joking. “It’s Leah who comes up with this totally inappropriate idea to look at adoption ads. She’s the one who sets the saga in motion. The whole story wouldn’t exist without her.”

Surely that sounds like a pile of highfalutin bullshizz, but Thirlby’s assurance in both herself and her roles is genuine. We’ve all met someone like this—the sort of New Yorker who exudes an unflappable self-confidence that borders on entitlement, yet who is driven for all the right reasons. Snow Angels director David Gordon Green clicked with her immediately and shut down a full day’s casting after the two met. “She’s self-aware without being self-conscious, exotic yet down-to-earth,” he says. “She’s really good at being herself and saying her own thing, and that brought out the best of everyone in the room.” It’s hard not to feel jealous of all that cool conviction. “That comes from my dad,” she admits. “He’s owned a construction company for 30 years, and problems arise every second, but he’s very centered.”

Born in 1986, Thirlby grew up at 6th Street and Avenue B. “My entire sphere of existence up to when I was 18 years old was between 6th and 16th,” she remembers. Of course, Alphabet City was quite a different ’hood back then. “My parents know they were absolutely insane to raise a baby there. But I wasn’t aware of how nice the neighborhood was or wasn’t.” Dad started his company in their building’s storefront. And Mom was an advertising executive who would leave ad copy along the lines of “Take Advil for your aches and pains” lying around the apartment. “I once took that into the bathroom to rehearse it and then I performed it for her,” remembers Thirlby. “I loved playing dress-up. As soon as I was old enough to say, ‘I wanna try acting,’ I did.”

She developed her chops in school plays (at Friends Seminary on the East Side) and the Shakespeare program at London’s Royal Academy of Dramatic Art. Early on, the seemingly unshakable youth discovered at least one thing that made her anxious. “I’d feel this debilitating nervousness the moment before I’d go onstage,” she recalls. “But you just step out and it goes away.” Movies aren’t the same. “There’s definitely no nervousness in film. You’re not performing for anybody at all, really. You’re just trying to be realistic.”

Scoring the United 93 gig during her senior year put a temporary kibosh on her school-play career—although it certainly moved her buzz beyond the auditorium. Maybe it’s because she’s a New Yorker, but one senses that no matter how much attention Thirlby attracts in the coming years, she’s simply too well-grounded to snap like some of her peers, or to generate a new gossip industry like some others. “Acting is not who I am, it’s just what I do,” she offers. And no, she’s not full of crap.

The Wackness screens at the Tribeca Film Festival starting Apr 26.

NEXT: Olivia Thirlby’s cinematic ascension »