Rose Byrne - A Rose that grew in the shade

After many bit-part roles in big movies, Rose Byrne tells James Mottram what it's like in the spotlight

There's something quite Zelig-like about Rose Byrne. Like the Woody Allen character, the Australian actress has been quietly loitering in the background of series of major movies. First there was Star Wars prequel, Attack of the Clones, in which she played a handmaiden (she's in good company; Keira Knightley played a similar role in The Phantom Menace). "I just had to stand behind Natalie [Portman] looking very demure," she purrs, in her soft Sydney accent.

There have been others: a Duchess in Sofia Coppola's Marie Antoinette; a crewmember in Danny Boyle's sci-fi hit Sunshine and an appearance in Matt Dillon's directorial debut City of Ghosts. None of them got her noticed. Even winning Best Actress at the Venice Film Festival didn't seem to help. It came in 2000 for her unstable blind girl in The Goddess of 1967, but the film slipped into obscurity.

Then came her biggest film to date, Wolfgang Petersen's glossy take on Homer's The Iliad, Troy. Considered for the role of Helen, Byrne wound up playing the lesser part of the slave-girl Briseis. It did mean a love scene with Brad Pitt – and, briefly, the limelight was on Byrne, who seemed to freeze like the proverbial rabbit. "I've got this tiny part in this big Brad Pitt movie," she said at the time, "and everyone here thinks I'm playing Sally to Brad Pitt's Harry – and I'm not. I'm constantly having to say that."

During this frenzy, she moved to LA to support the release of Troy as well as romantic thriller Wicker Park, in which she co-starred with Josh Hartnett. Needless to say, the laid-back Aussie didn't take to it. "It's pretty vacuous a lot of the time," she says of the city. "It's just that thing of when you go out, you feel like you're always networking. It's sometimes hard to have a conversation about something else. I don't know how you can be an artist trying to represent someone's life, if all you're doing is being involved in the film industry."

When we meet, the 29- year-old is in London with her "dear friend" Glenn Close to press the flesh for Damages, the hit US legal show that's in its second season on BBC1. If that wasn't enough, Byrne can also be seen this month in Knowing, an apocalyptic blockbuster in which she stars with Nicolas Cage.

Curled up on a sofa, Byrne's dressed in a formal ensemble – a navy pinafore dress and a tailored black jacket – that suggests she's come straight from the set of Damages, in which she plays Ellen Parsons, the naïve protégé to Close's scheming legal eagle Patty Hewes. The pale-skinned Byrne looks a little uncomfortable; as if all this being shoved to the foreground has taken its toll.

"I was a very shy little girl," she confirms, "and I'm naturally a timid person. I've got better as I've got older, but I'm usually shy and easily get intimidated, initially. It takes me a while to get warmed up."

None the less, it's a character trait that makes her perfect in Damages, which may be the only legal drama not to have any court scenes. "It's really about the lives of lawyers," says Byrne, who received a Golden Globe nomination for the first season. "My character is the eyes of the audience. She enters Patty's world."

In the second series, after discovering that her boss was responsible for an attempted hit on her life, Ellen turns FBI informant, assisting a criminal investigation into the firm. Byrne thinks the show is successful because of its ambiguous feel. "It looks at what is virtuous and what isn't. Being bad is not necessarily all bad, because you can get some good done. It's very much dealing with issues like that. The character of Patty is fascinating. She's so mercurial and duplicitous, yet she does some incredible things – but at what price?"

After time spent in England (making Sunshine and horror sequel 28 Weeks Later), Byrne now lives in New York, where she films Damages six months of the year. With a third series due to start rolling in the autumn, making Knowing proved something of a boon, as it took her back to Australia.

Directed by Alex Proyas, Knowing is a gloomy thriller tailor-made for conspiracy theorists. Cage is a teacher, who opens up a time capsule unearthed at his son's school to find a series of numeric predictions concerning various natural disasters – some past, some still to come. Byrne plays Diana, a single-mother that winds up on the run with Cage as he looks to warn anyone who'll listen that the end of the world is nigh.

With bodice-rippers, horror movies, sci-fi spectaculars and legal dramas already firmly on Byrne's CV, next up is an old-fashioned love story. Having premiered at Sundance this year, Adam is a romance between her and British star Hugh Dancy, who plays a sufferer of Asperger's Syndrome. "We develop this unlikely affair. It's a very sweet, simple story – but it's very funny and tender and poignant."

As for her own love life, as she puts it, "I've been with the same man forever." Well, five years to be precise. He is the Australian actor and playwright Brendan Cowell. They've yet to work together, though Byrne insists this is not by design. "He works a lot in Australia, and I guess I've been over here a lot. But one day ... our time will come." As for Byrne, whether she likes it not, it seems her time is already here.

'Knowing' opens on 25 March. 'Damages' is on BBC1 on Sunday evenings