Take The Bourne Ultimatum in which Matt Damon returns as the trained assassin Jason Bourne who finally gets an answer to how he became what he is . . . but in doing so is seen as a threat to American security: he has to be taken out no matter what the collateral cost in innocent civilian lives. Director Paul Greengrass depicts a Bush-era intelligence service out of control and corrupted by power, ready to turn its global surveillance and counterterrorism powers . . . including abduction and murder . . . on the people it's supposed to protect.

"Paul elevates the story and gives it a depth beyond what you'd expect from an action thriller, " says Stiles, who plays Nicky, an agent within the CIA who questions what is happening and wants to call a halt but in doing so becomes a target herself. "Nicky has a conscience. She realises she's part of something that's morally suspect, as does Bourne. What's interesting about him as an action hero is that we're not showing him as being cool for killing somebody. He's suffering from what he's had to do."

Julia Stiles was already an experienced stage actress when . . .at 17 . . . she made her screen breakthrough in 10 Things I Hate About You, a teen comedy version of The Taming Of the Shrew. She's been Ophelia to Ethan Hawke's Hamlet, won an MTV award for Best Kiss in the inter-racial romance Save The Last Dance and exacted vicious sexual revenge on her domineering boss Stockard Channing in The Business Of Strangers. Something of a muse to David Mamet since he cast her in State And Main as a teenage girl who catches the eye of Alec Baldwin, a film actor with a weakness for underage girls, in 2004 she starred on the West End in his controversial play Oleanna protraying the student who accuses her professor of sexual harassment.

Still only 26, and a graduate in English Literature at Columbia University, Stiles is self-assured and articulate but with a certain East Coast reserve, a cool blonde in the Grace Kelly manner. Like Kelly, she too has Irish roots . . .despite her name. Her father is a New York teacher and business man, John O'Hara. "I've always been Julia Stiles because that's the name on my birth certificate, " she says. "My mom didn't change her name when she got married, so my end name is her name and my middle name is my dad's . . . not very Catholic or Irish, I'm afraid."

She doesn't conform to an Irish American stereotype. "I'm not in touch with anyone in Ireland . . . it's a generation or two removed. My dad comes from Queens, which was a classic Irish-American Catholic community and had a big influence on him, but I grew up in Manhattan." Instead of being educated by nuns, she went to a Quaker school. "My dad is a lapsed Catholic. My mom isn't a Catholic. I'm still trying to figure out what religion I subscribe to."

Her first speaking role was as the daughter of Harrison Ford, an Irish American cop who unknowingly shelters IRA-killeron-the run Brad Pitt in the 1997 Alan Pakula thriller, The Devil's Own. "But it wasn't an ambience I was familiar with from growing up. My childhood wasn't particularly national. Manhattan is very integrated, it's a melting pot."

Irish director John Moore brought her to Ireland in 2005 for some re-shoots for his remake of satanic horror thriller The Omen.

"There's a park scene where the boy Damian gets lost and I am screaming for him, a distraught mother, and that was shot in Dublin. So I did get to try an authentic Guinness.

She started acting at 11, more by chance than anything else. "My mom is an artist, but not anything in the performing arts . . . a potter, " she says. "For me it was really the fact that, growing up in SoHo in New York, I didn't have a backyard, so my pastime would be to play dress up. I would watch a lot of TV, unfortunately, and would act out what I saw. I was kind of a ham and was lucky enough to be around this group of people who had a theatre company and they needed a child to be in one of their shows and ended up being a regular. I just loved it. I loved being on stage. I loved performing. I loved pretending to be a different person. And then that turned into what is now a career."

It wasn't just any theatre company, it was the highly experimental MaMa Theatre company. "A lot of it was about timing. For instance, three actors would be on stage playing the same character. We would have pre-recorded dialogue and we'd all have to be acting in unison, even with sound effects, so it actually was very musical and good training in terms of being disciplined and technical."

MaMa conditioned her to see that there are many facets to people: no one is ever exactly how they seem. Acting is an undermining of stereotypes and pat assumptions. "I'm definitely drawn to what is unexpected, " she says.

In Mona Lisa Smile, she stars with Maggie Gyllenhaal, Kirsten Dunst, Juliet Stevenson and Marian Seldes as a group of students in Wellesley College in 1953 whose bohemian art teacher Julia Roberts tries to persuade them to have careers rather than just settling for just becoming a wife and a mother. "I was happy when my character actually did make a choice not to pursue a career in art but to get married, because I think that was realistic.

Coming from a perspective after the women's rights movement, you can lose track of where in history that happened. Today people can pursue a career or pursue a family or both if they want to . . . not that it's easy . . . but it was different in the 1950s."

Stiles was quite the opposite to sweet and loving in The Business of Strangers, playing a young temp who accuses a womanising man of having raped her in the past. "I was happy to have a chance to play a kind of sinister character. That was a departure for me . . . again living out something on the screen that you would not otherwise in real life."

She had no plan when she made acting a career. "I think some of it has been a little bit trial and error, just a learning experience for me, but I'm slowly starting to feel I'm more in control and more in the driver's seat. I've managed to work in certain genres that definitely have conventions but I've tried to do what was unconventional. So although Bourne is an action thriller, we've tried to make it as original and unexpected as possible. We tried to make The Omen better than just a slasher film, something that really is psychological and strikes at basic fears that we have."

She's wary of attempts to group her with other actresses as a new wave of strong women in Hollywood. "There've always been strong women in Hollywood who've made their presence known. Hepburn and Bacall were really popular in the 1940s and 1950s because of their feistiness and their strength and the rapport they had with their male co-stars.

I've a hard time trying to pay attention to the film industry as a whole. I tend to think more personally about my career and the stories I want to tell and the experiences I want to have when I go to work every day. I think the only difference nowadays is that a lot of actresses are getting pregnant and that doesn't seem to ruin their careers. If anything they seem to have great careers after they become mothers, which I don't think happened 50 years ago."

What's changed today also is that there are now more women writing screenplays and directing and producing. Stiles made a move behind camera with her short 20minute film Raving, which she wrote for the Ellemagazine film series. It was shown at this year's Sundance Festival. "You can download it on iTunes. It's about two strangers in New York . . . a rebellious young woman played by Zooey Deschanel and a lonely disorientated older man Bill Irwin . . . and how they meet randomly and change each other's lives. It was a great way for me to learn so much about the whole process of film-making from start to finish.

But I love acting. I never want to give that up."

She's also a producer and is in pre-production on an adaptation of Sylvia Plath's autobiographical novel The Bell Jar. "I've been trying to get the rights for a long time and now finally a screenwriter is adapting it and a production company in New York is producing it with me. The most important think is to get the script right because I've a very clear vision for it and I want to get it on paper."

As with Mona Lisa Smile it's set in the 1950s and centres on a young book editor . . . played by Stiles . . . who becomes troubled by the prevailing social attitudes and slips into mental illness. "We've come a long way since the 1950s but there are definitely parallels between that decade and what's going on now. Life seemed so safe and mannered and structured then, so you're always intrigued about what's going on under the surface." Politically the 'redsunder-the beds' hysteria and witch-hunts have parallels with today's so-called 'war on terrorism'. "History comes in cycles, " she shrugs.

She's not worried that Plath is a cult figure about who many people may feel possessive.

"That's one of the reasons I want to make The Bell Jar into a film.

Her biography has overshadowed so much of her work. People are more obsessed with her suicidal fate than what she actually wanted to contribute in terms of being a writer. I think The Bell Jar and her poetry have many beautiful hallucinogenic images that are perfect for film."

Stiles will next be seen with Angela Barrett and Danny Glover in Gospel Hell, set in a small town in the south where long before a murder took place but wasn't prosecuted. "I play a school teacher who's very bright-eyed and comes to the town thinking she can change the world, but realises very quickly that the town has an ugly history."

Stiles votes Democrat. She supported Kerry for president.

She works for Amnesty International highlighting the harsh conditions of immigration detention of unaccompanied juveniles. "Films are entertainment but beyond that level they are also a forum for communication of ideas and provocation, " she says. "We've an obligation to talk about relevant issues. Audiences now expect something that's going to resonate with them more deeply and not just be popcorn films."