When she was 16, Thirlby started doing commercials, including a Clean & Clear spot in which she had to be made up with artificial zits. Her feature-film debut was inauspicious: The Secret, a family drama costarring David Duchovny and Lili Taylor, never opened in the U.S. It was a small part as a passenger in Paul Greengrass' United 93 that got her noticed. ''When my managers first approached me with the idea of being in a 9/11 film, I was offended,'' says Thirlby, who was at school in New York when the attacks happened. ''Then I realized that being part of United and making the film was actually such a positive contribution.'' Thirlby followed United with a small arc on the short-lived NBC series Kidnapped, then landed a part in David Gordon Green's drama Snow Angels (which came out in March 2008). Finally, in January 2007, she scored that role as Juno MacGuff's best friend. ''I was continually impressed by how good of an actress she was,'' says Juno director Jason Reitman. ''It was a very big question of who's going to play Leah, who's going to have this chemistry. I put [Page and Thirlby] in a room together, and they got along so well.'' As Page remembers it, ''We started reading the scene and it felt so natural. There was just a sense of feeling comfortable. We became really good friends.''
Back in Central Park, Thirlby has managed to get lost. She's trying to find the exact spot where she and her Wackness costar Josh Peck (of Nickelodeon's Drake & Josh fame) filmed their first kiss, one of the film's pivotal moments. Finally, she finds the rock slab where she and Peck smoked (faux) pot and made out under the branches while nearby real kids toked actual ganja. Set in early-'90s New York and accompanied by a booming classic hip-hop soundtrack, The Wackness (costarring Ben Kingsley, Mary-Kate Olsen, and Method Man) is a stylish coming-of-age story in which Thirlby plays a cooler-than-cool high school temptress who's the object of drug dealer Peck's affection. In addition to lots of funny, naturalistic banter, the pair share a series of painfully awkward deflowering scenes something Thirlby also depicts in Snow Angels and New York, I Love You. ''I actually was being interviewed once and the interviewer was like, 'So is that going to be your new thing: taking people's virginity?' At the time I was, like, totally offended. And then I realized, Oh my God. That's kind of it exactly.''
It's unclear whether Thirlby will be bedding more virgins on screen soon. After 2009's Lauren Graham-Jeff Daniels romantic comedy The Dream of the Romans, she doesn't have any projects lined up. But the offers will almost certainly soon start pouring in, and the indie darling will have to make some hard decisions about what kind of actress she wants to be. Though she auditioned for (and didn't get) the role of Wonder Woman in the upcoming Justice League movie and wanted to play Trixie in Speed Racer, Thirlby remains somewhat wary of Hollywood. ''I think it would be nice to find something you could use as a foothold to climb,'' she says of big studio films. Still, ''I have no interest in becoming really famous. It's easy to forget sometimes that it's a fairly stupid profession. It's basically glorified make-believe. It's always been my passion, but let's face it: I'm doing the same thing I was when I was 3 years old and getting tutus.''
The sun's going down in the park, but Thirlby is having too much fun people-watching to head home.''I love the summertime,'' she says. ''It's what Josh says at the beginning of The Wackness: 'With all the ladies and their sho' skirts and their breastesses.''' Finally, she's ready to leave, but as she walks toward Fifth Avenue something catches her eye. ''It's Alice,'' she says, referring to a fountain sculpture inspired by Lewis Carroll's Alice's Adventures in Wonderland. Thirlby is delighted by the discovery, perhaps because she, too, is a young girl about to embark on wonderful adventures by falling down the rabbit hole.