Love, hate and rows with Bjork

Secrets are a theme of Lars von Trier's award-winning film, Dancer in the Dark. But, as Charlotte O'Sullivan discovers on meeting the Danish director in Copenhagen, it's the secret his mother kept from him until her death that holds the key to his work

I'm perched in a doorway of an old army barracks in Copenhagen, disturbingly reminiscent of a set from The Avengers. I'm waiting for Lars von Trier, whose production company, Zentrope, is based there. I hear squealing tyres and a red open-top sports-car whizzes past. It contains von Trier and his friend, Festen director Thomas Vinterberg, both grinning from ear to ear as if to say "aren't we naughty!". I am escorted into Von Trier's office, where I head straight for the comfy-looking, sun-dappled sofa. "Sorry," says his assistant, "Lars prefers to sit on the couch."

I'm perched in a doorway of an old army barracks in Copenhagen, disturbingly reminiscent of a set from The Avengers. I'm waiting for Lars von Trier, whose production company, Zentrope, is based there. I hear squealing tyres and a red open-top sports-car whizzes past. It contains von Trier and his friend, Festen director Thomas Vinterberg, both grinning from ear to ear as if to say "aren't we naughty!". I am escorted into Von Trier's office, where I head straight for the comfy-looking, sun-dappled sofa. "Sorry," says his assistant, "Lars prefers to sit on the couch."

Von Trier then appears, yanking up the front of his trousers and wearing a crumpled, slightly potty expression. I expected weird from the brains behind the groundbreaking Dogme-95 movement. I expected different from the soul behind some of the best films of the last 10 years. How could one expect less from the man who gave us Breaking the Waves, with its red-raw melodrama; or The Idiots, a punch-drunk, extremely graphic, inner-child fest; or my own personal favourite, the existentially wired hospital soap, The Kingdom, in which Von Trier himself used to appear, over the end credits, wearing a natty waistcoat and making the heavy-metal sign of the devil.

But I didn't expect this. To customise the famous description of David Lynch, Von Trier looks like a Benny Hill from Mars.

As it happens, the 44-year-old Danish director seems quite happy to play up to this image. During the shooting of his latest film, the Palme d'Or-scooping, audience-dividing Dancer in the Dark, Von Trier and his star Björk famously fell out. Von Trier accused her of eating her costume. She recently charged her director with being an "emotional pornographer".

"I'm flattered!" he says, now ensconced in the sofa and plucking at the front of his trousers once more. "I'm pro-pornography, so those words don't bother me at all - pornography is showing things as they are!" We then have a little row about pornography, during which he wags his finger at me and says "that's a very feminine way of looking at it". I begin to feel more than a little sorry for Björk. Von Trier says they were very mean to each other during the shoot. His one regret? "That I wasn't meaner!"

And then his mood changes. Björk claims that Von Trier surreptitiously had her filmed for the purposes of a behind-the-scenes documentary. And asked about this, Von Trier - for the first time - lets the wind-up routine and the soundbites go.

He says it was never secret, that they got her permission according to the rules of her contract - it was only later that she said no, at which point they stopped filming her. What really annoys him, however, is that it now looks more than likely that the director of the documentary is going to have to take out the bits that she was in. Which will, of course, leave a lot of holes. Clearly, Von Trier feels Iceland's righteous little pixie should be more honest about her legal clout. But there's more to it than that.

"We really disagree about this," he exclaims with a sigh. "I was fed up with all this secrecy when I was younger. All this need for secrecy in art - it's about making yourself more mystical, and more fantastic, and making other people the opposite. I understand her point of view (it seems even mentioning Björk's name has become painful to him), she thinks that you shouldn't talk about the religious act. You're supposed to think everything happens as a result of divine inspiration, but that's so wrong."

All of which heretical claims for openness sound very impressive, until you remember that Von Trier is a born-again Catholic (a religion, surely, which hugs mystery to its bosom). And has just made a film boasting a deeply Catholic sensibility. Slowly going blind, Dancer in the Dark's heroine, Selma, is tortured by the idea that her son, Gene, will one day inherit her disease. To ward this off, she saves money to pay for an operation that will cure him, but tells him nothing about her plans, ostensibly in order to prevent him from worrying. She also keeps the secret of a neighbour, Bill, who wants to hide from his wife the fact that he's broke. She keeps these secrets with a determination that is positively frightening and yet she's seen by almost everyone around her as a wonderful martyr.

"You don't really like Selma, do you?" says Von Trier with a smile.

Well, I say, ideals are fine but I wouldn't want her for a mother. "But that was my mother!" he says, almost choking on a chuckle. "My mother was an idealist, a very strong woman. As a child, I had the magic feeling that whatever she said would come true. But when she died, I found out that my father was not my father. And she kept this secret to protect this man and his family, which must have been terrible for her."

He leans his head back on the pillows, getting himself comfy, and it's at this point that I realise why he prefers the couch. He's in the mood for spilling secrets. The therapy session has begun.

Von Trier's mother was a communist, who had no qualms about "screwing around". But honesty did matter ("I used to ask my mother, 'promise me I won't die tonight' and she would say 'I can't promise you that' "). But for some reason, in this case, she decided Von Trier couldn't be trusted with the truth. She had the affair while she was married to Von Trier's stepfather and only told Von Trier about it when she was dying.

He says he wasn't angry with her for taking so long to break the news, then changes his mind. "It was very, very stupid not to tell me. Why spend a lot of hours on the sofa with a psychiatrist, trying to find out things about your childhood, when quite an important fact is missing!"

It's all just pouring out now - there's nothing I can do to stop the flow of intimacy. "When she told me," he continues, "it was two days before she died, with all these tubes up her nose. It was the last time that I talked to her, and she said: 'I was looking for some artistic genes for you - you should be quite pleased.' [Von Trier's biological father comes from a long line of composers.] And I remember saying to her, 'If this is a line from Dynasty, it's a really poor one'."

I suggest that that was a bit cruel and he makes me jump as he roars, "Oh, come on!" On the night she died, he went home, gathered together all her favourite little glass ornaments and smashed them on his concrete floor.

"She believed what she was doing was the best for me," says Von Trier, "but it was a betrayal. She put him in front of me. She was crazy about him. I read her diaries - for 10 years, she was..." he flutters his hands, "but I think that was because he was an arsehole. You know, women tend to like the arseholes!"

He doesn't look up at me when he says this. He's not trying to provoke me, now - I'm just part of the furniture. It should be alienating, but it's not. It's cosy.

His father really doesn't sound very nice. Von Trier met him just after his mother died, but his father was, to put it mildly, uninterested in developing a relationship. He told Von Trier: "I never accepted that son, I always expected your mother to protect herself." He also said that if they needed to talk, they could do so through his lawyers. "He didn't want to know me!" yelps Von Trier. "My mother said 'Oh! He's such a nice man. You will love him', and I didn't at all."

Of course, as ever with von Trier, it's not quite that simple. I ask if he looks at all like his father and he immediately pads off to look for a picture he has lying around. He spends around five minutes looking, but in vain ("Where are you, daddy?" he mutters pointedly to himself). So, clearly, he is still interested in the "arsehole", who, by the way, was a Catholic (aha, so that's why he converted). Nor did he defy his father's love of secrecy, promising not to tell anyone whose son he was until his father had died (his father died two months ago).

Essentially though, Von Trier's heart still belongs to his stepdaddy, who died 10 years before his mother. He talks about his generosity, his Jewishness, his comforting lack of neurosis and low-key humour. "My mother said 'We think that he knew [about the affair]...' " - here he adopts a mimsy voice - " 'but he didn't say so much'." To which Von Trier metaphorically yells back: "Then talk for Christ's sake, bitch! Say something!" He shakes his head. "My [step]father was a very weak man in relation to my mother - he was very much in love with her and never looked at another woman all his life. He was a softie."

And so the scenes from the melodrama (his word) fall into place. Von Trier has a history of depression (he took to Prozac a few years ago). And I imagine that, for all the pain, it's rather nice for his angsty gums to have something hard to chew on. "Oh yes," he says, "I always envied people like Bergman who were mistreated as children, well now I'm fine, aren't I?"

More importantly, Dancer in the Dark begins to make a little more sense in the wake of these revelations. Critics have been busy drawing links between Selma and Björk (who also has a young son, roughly Gene's age). But clearly, the film is even closer to Von Trier's home: a mother who keeps her son utterly in the dark to protect his artistic "vision"; a mother who keeps a bad neighbour's secret, in order to maintain his position as a respectable, married man. And that final detail: Dancer's sweet dork of a guy, Jeff, who trots loyally after Selma despite her obvious lack of interest, otherwise known as Von Trier's gentle stepfather - the man to whom the director owes so much (even, but not especially, his name).

I'm beginning to understand, too, why Dancer elicits such different reactions: it's meant to. Von Trier himself both loves and hates Selma, so he's made sure that some of us love this secretive, magical woman, and some of us want to wring her neck.

Even the row with Björk has a role to play. Whether fuming about the documentary and Björk's need for "secrecy", or moaning about her childish desire to "take the easy way out", Von Trier's language in relation to her is deliciously significant. Von Trier describes the pair of them as a married couple "who had our lawyers with us all the time, to prepare the divorce". Might a therapist not suggest that through Björk he's been trying to divorce his "bitch" of a mother on behalf of his "softie" stepfather? As he says, "I've had enough of being a softie!"

Selma/Björk/his mother - they've all merged into one in Von Trier's mind. But the process seems to have been productive. "I've been quite down," he says, "after this encounter with Selma and Björk. But just this morning, I've been thinking about a new idea for a film in a positive way. It's a very, very black idea. Finally in one of my films, we'll see the woman as the real arsehole. I have so much inspiration," he says, with a mischievous roll of his eyes, "but I'm not saying whether the inspiration comes from my mother or other people."

So should we be looking forward to Dancer in the Dark - Part II? His blue eyes narrow: "A boy pissing on his mother's grave - it's a good image, isn't it?"

'Dancer in the Dark' is released on15 September