Julie Delpy smokes and smokes. She smokes so much she should consider wearing ashtrays as trinkets. We are in a faux-rustic restaurant in Berlin's Potsdamer Platz, where she is promoting the sharp and funny 2 Days in Paris, a romcom with bite, which she wrote, directed, produced, edited, scored, starred in and presumably smoked all the way through making. At first I think the only time she's not smoking is when she's talking. But she has so much to say - she can rattle off an entire exhaustive answer before you've finished asking your initial question - that eventually the division between speaking and inhaling vanishes, and the words are tumbling from her mouth wreathed in smoke. It doesn't sound very attractive, but remember this is Julie Delpy. She could be up to her elbows in offal, belching the Marseillaise, but she'd still have admirers establishing cults in her name. And chances are she'd still loathe that kind of attention.

"I hate being a male fantasy," she spits. "So many times I've been in a room pitching some movie to the financiers, and they're blatantly just staring at my legs." Before this conjures images of a stereotypical Hollywood executive leering from behind his desk, Delpy is quick to point out she won't take that behaviour from revered arthouse auteurs either. In the early 1990s, she auditioned for the dual lead roles in Krzysztof Kieslowski's The Double Life of Veronique. "He asked me to do a sexy gesture," she says, her incredulity undiminished after all these years. "That really bothered me. So I did this ..." She pokes her tongue out and tugs on her earlobes like a playground brat. "I knew by the look on his face that I hadn't got the part. But I was really mad with him. All that younger-woman bullshit you get. That fucking pervert. That... man!" She makes the noun drip with derision.

Delpy and Kieslowski eventually worked together in Three Colours: White (1994), in which he cast her as a woman who divorces her husband when he fails to consummate their marriage. And the pair enjoyed a friendship - Delpy now says that the late director was "the greatest man in the world". "I think my audition for Veronique convinced him I could play someone a bit hard to handle. Not that I'm bad-tempered. But I have my pet peeves. I always hated, when I was growing up, all these directors who wanted to be my Pygmalion." (...)"I like to be the Pygmalion, instead of someone else," she decides. "I prefer to make people into things, not be moulded by other people."

The most interesting thing about the 37-year-old Delpy in person is her unexpected hardness. I suddenly realise she's never shown it fully in any of her performances. Yes, there was a hint of it in Three Colours: White. But while she is sublime in her signature role, as the idealistic Céline in Richard Linklater's Before Sunrise (1995) and its sequel Before Sunset (2004), she only gets to play the odd prickly moment. Even in An American Werewolf In Paris (1997) - which, like her brief spell in TV's ER, was one of the few mainstream concessions in a stubbornly marginal career - she is a smiling lycanthropic temptress, never quite dredging up the darkness necessary to tip that film from comedy into horror. The woman herself is another matter. Beneath the playful indiscretion and hearty laughter is something flinty, businesslike and brusque.

It's all the more fascinating because she was out in public a few nights earlier, putting on what I now realise was a performance. After a rapturously received gala screening of 2 Days in Paris, she stumbled up the red-carpeted steps, then blathered absent-mindedly into the microphone. It was nothing like the focused, clear-sighted woman sitting before me. Possibly she felt there was spikiness enough in her film, and that the audience that night deserved a taste of Brand Delpy, with its inbuilt promise of gap-year goofiness.

(About 2 days in Paris) A rather worrying motif of castration and dismemberment runs through the film. "The truth is it was written and directed so quickly, I didn't have time to analyse it. Yes, there is talk about chopping off penises, condoms cutting off circulation, penises needing to be lifted with balloons, so I have to say ... No comment." She seems genuinely bashful, caught out by the subtext of her own film. "I scare myself. I think I have a penis obsession. Because I don't have a penis. Well, I mean, I have one at home. But it's not on me. It's attached to my partner. Maybe I wish I had one ..." She takes a few urgent drags on her cigarette, wafting away the smoke, and the subject.

(On the differences between "2 days in Paris" and "Before Sunset") "I twisted the idea of Before Sunset. I love that film but the characters are in a bubble; Paris has no influence. Here, it's the opposite. The city is destroying Marion and Jack - their environment is literally attacking them."

"I love acting," she says, "but I'm not a very actressy person. I don't like the vanity it encourages, the way it makes you concerned about your age or your appearance. Writing doesn't have that problem. For me, writing comes - well, I wouldn't say before love but ..." She thinks for a moment, gives a little wobble of the head and changes her mind. "Yeah, I'd say it comes before."