She is openly bisexual, and was married to Jacques Pienaar, a sound engineer whom Thirlby met on the set of Dredd. Pienaar filed for a divorce in March 2021 though. She is best friends with Zoë Kravitz.
- United 93 (2006) as Nicole Carol Miller
- Juno (2007) as Leah
- The Secret as Samantha Marris
- The Wackness (2008) as Stephanie Squires
- No Strings Attached (2011) as Katie Kurtzman
- The Darkest Hour (2011) as Natalie
- Dredd (2012) as Judge Anderson
- The Stanford Prison Experiment (2015) as Christina Maslach
- The Wedding Ringer (2015) as Alison Palmer
- Above the Shadows (2019) as Holly
- The White Orchid (2019) as Claire
- The L Word: Generation Q (2019-20) as Rebecca Dowery
- Y: The Last Man (2021) as Hero Brown
- Oppenheimer (2023) as TBD
Tropes in her work include:
- Big Ol' Eyebrows: She has very prominent dark eyebrows. These make her characters' faces more noticeable, in contrast with her pale skin, thin face and small eyes.
- Dawson Casting: She started out playing a lot of teenage characters, while around 20 or 21.
- Dyeing for Your Art: She dyed her hair a metallic blonde to play her character in Dredd, Judge Anderson, though it had to be continually re-dyed during filming as her roots kept showing. Other roles had more minor examples, where it's made slightly lighter with highlighting.
- Hollywood Hype Machine: She was billed as one of Hollywood's "New Wave" by Vanity Fair in 2008, but has had a pretty modest career since then.
- Nice Girl: A large number of her characters are sweet, kind women.
- Playing Against Type: Her role in Dredd as Judge Anderson was quite unlike any before or since, as she's an action girl with an arc of growing tougher over the film. Virtually all of her other roles are decidedly non-action oriented, with most characters being much gentler (though Anderson is still a Wide-Eyed Idealist starting out).
- Queer Character, Queer Actor:
- Raven Hair, Ivory Skin: The looks of all her characters, given that she has them naturally (except in Dredd, when it's dyed a very artificial blonde). In almost every case, they're depicted as beautiful (not hard, again, as she's already good-looking).
- When She Smiles: Due to her having a very charming grin which shows all her teeth, the characters she plays in darker roles look very lit up if they smile.