Topping her performance in the 1995 romance Before Sunrise was never going to be easy, but Julie Delpy - and her co-star Ethan Hawke - took a hands-on approach to the script of the sequel, Before Sunset, and the result is even better. She talks to Donald Clarke about staying in control of a career she loves

'Oh, I just can't stand pretentious people," Julie Delpy snaps in her chocolatey Franco-American drawl. Really? I suppose I shouldn't be surprised. But she was so convincing as the relentlessly self-analytical (and pretentious) Celine in Richard Linklater's crafty 1995 romance Before Sunrise that it was hard to stop oneself jumping to the conclusion that she was revealing little bits of the inner Delpy. Then again, in Before Sunset, the even more brilliant sequel, she appears as a less precious, more realistic Celine. Perhaps age has seasoned both the character and the actor.

Nine years after the delicate blonde intellectual spent a magical evening strolling the streets of Vienna with Ethan Hawke's equally bookish Jesse, the two go for another amble (on Bloomsday, according to a sharper-eyed critic than me) round a heart-stoppingly gorgeous Paris. They had originally arranged to re-unite six months later, but Celine was unable to make the date and is taken aback to discover that Jesse has now written a rapturous novel about the evening.

"When you are in your thirties you suffer less with love because you have a sense of humour," Delpy says when I ask her to talk about the changes through which both she and Celine have gone. "Before, my criteria for a man was that he live up to some perfect ideal. Now my main criteria is that he have a sense of humour; that means good times ahead. Actually, I just get dirtier as I get older." Dirtier? Mop my brow please, nurse.

Despite occasional moments of rambling eccentricity ("I felt The Day After Tomorrow had a purity"), the 34-year-old Delpy proves to be extremely good company. Wide-eyed and wild-haired, she relishes the opportunity to batter any question into submission with colourful anecdotes and strident opinions. She does, nonetheless, insist that when at home in LA she is basically a tranquil sort. "I really need to have space to be quiet. I am talking a lot now, because I am in an interview. And I have just done this film where I talk all the time, but actually I love moments of peace."

Delpy was born in Paris to parents who were both actors. It sounds like a complicated childhood. Though a glance at the Internet Movie Database confirms that both Mom and Dad worked a great deal, Julie, a sickly child, spent a brief period in care when they couldn't pay the bills. She thought of going into politics or being a writer. Then mad old Jean-Luc Godard popped up and cast her in 1985's Détective, one of his more coherent later movies, and there was no turning back.

She went on to work for an impressive array of the world's great directors - Agnieszka Holland, Mika Kaurismäki, Bertrand Tavernier - but many viewers will always think of her as Dominique in Krzysztof Kieslowski's Three Colours: White (1994). She has, however, never quite managed to translate the iconic status that film accorded her into mainstream stardom.

"I tried the Hollywood thing and the way I tried it I didn't like it," she says cryptically. "I can't just be doing some big dumb movie. I did American Werewolf in Paris and you know it just didn't do it for me. Some of those movies I like. But I just can't do the big dumb ones."

Before Sunset is certainly neither big nor dumb. But it could so easily have proved to be a dreadful mistake. One of the things that viewers savoured about the first film was its ambiguous ending - would the two lovers, strangers until a few hours previously, meet up in six months or would they never see each other again? If the new film, on which both Delpy and Hawke receive a screenwriting credit, took a wrong turn it could retrospectively sully the reputation of its predecessor.

"Oh I know!" she says. "We were mailing these scenes to one another and Ethan was writing these lovely monologues and then just two weeks before we were to shoot we watched Before Sunrise and I was very depressed and freaked out. We can't screw this up. So we went through the script and cut out the bullshit. Anything that wasn't important to the development of the characters we cut out. Even if there is still a lot of intellectual blah-blah, we cut out a lot more intellectual blah-blah."

The two actors had first revisited the characters in 2001 for Linklater's fascinating animated reverie Waking Life. "What would Celine and Jesse do if they had a dream of reuniting? They would keep on talking. They can't stop even in their dreams. In a way, Rick always felt that something was missing after Before Sunrise, and after Waking Life he still felt there was something missing." Delpy claims that there were very few arguments between the three main players as to what the characters would have gotten up to over the last decade. Indeed, the most significant dispute seems to have taken place during the making of the first film. Delpy believes that this earlier discussion revealed a lot about the differing ways men and women view romance.

"I wrote that they should meet six months later, but the guys thought they should just go away and never see each other again," she says. "But I just thought that, as a woman, I need some opening or hope to romanticise about, because that is just how women are. I like to think that something is never quite over until your life is over." Which brings us nicely onto the tantalising possibility of a third instalment of the Jesse and Celine story. Mind you, given the perfect symmetry of the two titles, what on earth could such a film be called? "We have joked that it could be called Before We Go Crazy," she laughs. "But now the stakes are even higher. On the second one we were worried about destroying the first. Now we are worried about destroying two good films."

And what happens in such a film would surely depend on what happens to the two actors in the interim. Hawke has admitted that the break-up of his relationship with Uma Thurman affected his contribution to the script of Before Sunset and Delpy has said elsewhere that she allowed Celine, a political activist, to accomplish some of the things she wished she had managed in her own life.

Yet Delpy has done plenty. She recently finished a film course in New York University and is gathering the finance for a feature on the life of the legendary vampire Countess Bathory. She has released an LP of airy acoustic songs, one of which appears in Before Sunset. And she still finds time to act now and then.

"It may sound corny, but my idea of success is to follow your dream," she says. "I have always wanted to write and I always write. I may not have the typical life for an actress that others wanted me to have, but I have followed my dream more than others. That is a true success and I don't care about anything else."