American actor Olivia Thirlby knew the unwritten screen dictum didn't apply to her when she took the role of a young female law enforcement officer in the dystopian comic-book adaptation Dredd 3D, and her rationale is simple but sound.

''The proof that my character was never intended as eye candy is that they hired me - I'm not an eye-candy kind of girl,'' the 26-year-old says. ''They took pains to keep me out of a sexy or revealing costume because it wasn't a movie about that. It had to have an element of realism to it. They wanted these cops to look like riot cops and they wanted their uniforms to look like they would perform in the field.''

A New Yorker whose career has taken her to Los Angeles, Thirlby has steadily built a career from supporting roles in everything from 2007's Juno - that was her declaring ''Honest to blog!'' as the best friend to Ellen Page's pregnant teen - to romantic comedy No Strings Attached, in which she played Natalie Portman's younger sister. Now, a nascent career built mainly on offbeat comedies and low-budget dramas is getting a bloody serve of sci-fi-infused violence.

Released yesterday, Dredd is a fairly faithful adaptation of British comic book Judge Dredd, which made its debut in 1977. It has no connection to the dismal 1995 movie of the same name that starred Sylvester Stallone, Diane Lane (who is soon sighted getting changed in the film) and, supposedly as comic relief, Rob Schneider.

Permanently in a helmet, his eyes obscured, the latest Judge Dredd, played by New Zealander Karl Urban (Star Trek), is an update of Clint Eastwood's Inspector Harry Callahan, a monotonal judge, jury and executioner in an endless urban sprawl that barely houses the futuristic remains of America. The character, whose excesses turn the civic fantasies of law-and-order campaigners into blackly funny retribution, doesn't do one-liners, or much of anything besides zealously enforce the law.

''This movie was made by Dredd fans and it was made for Dredd fans,'' Thirlby says. ''In his 30 years of life, Judge Dredd has never had a romance and never taken off his helmet, so that's a hard character to adapt to film.

''If you're going to do that, you have to stay true to him. You can't go against the traits that have given him his iconic status. I appreciated the decision to keep romance and sex, as well as Karl's eyes, out of the movie.''

Dredd 3D is, in essence, a very expensive British independent production, directed by Pete Travis (Vantage Point), adapted by author and screenwriter Alex Garland (28 Days Later, Never Let Me Go) and shot by Anthony Dod Mantle (Slumdog Millionaire, Antichrist). It was shot on location in Cape Town, South Africa, and has a grimly propulsive energy.

It is Thirlby's job as Anderson, the rookie Judge assigned to Dredd, to provide a human perspective to the cruel future when the two armed dispensers of justice are trapped inside a 200-storey slum tower controlled by Lena Headey's beatific psychopath and crime boss Ma-Ma. With Dredd exiled from emotion, the thematic conflict comes from female characters: Judge Anderson, a psychic, can't help feeling everything, while villain Ma-Ma is the exact opposite.

''What Anderson is trying to teach people is that it's good to feel more,'' Thirlby says. ''The more you feel, the more you know. A lot of people, they shut down after they experience emotional pain and they never want to go through that again, but what Anderson is working on is this notion that if you feel everything, you're completely covered.''

Empathy is an actor's lifeblood, and it's allowed Thirlby to capture increasingly diverse roles. In the US, she's starring in Dredd as well as Ry Russo-Young's independent drama Nobody Walks, in which she plays Martine, a young filmmaker who arrives at the Los Angeles home of a sound engineer (The Office's John Krasinski) and calmly destroys his marriage without accepting responsibility.

''I want movies I haven't seen or done before. Something challenging, different and fun - I'm open to anything,'' Thirlby says. ''I go where the material is, and I feel like I'm looking for strong directors. That's the crucial ingredient.''

That approach means that Thirlby has divided her time between promoting Nobody Walks, co-written by Girls auteur Lena Dunham, at the Sundance Film Festival and taking the stage before thousands of rabid Dredd geeks at conventions. The contrast would sound bizarre if it didn't encapsulate the career she always desired.

''Acting is something that I always wanted, but I never paid attention to the notion that it might actually work out,'' Thirlby says. ''You have all sorts of ideas about what you want to do - at one stage, I wanted to be a jockey - but this is the one that's a big deal. My ultimate fantasy and my ultimate passion is acting, so I never let myself believe it would come true.''