The odd couple

The idea of the outsider is a consistent theme in his films - and in von Trier's life. The son of middleclass communist academics, Lars Trier grew up an activist, and at 12, sped to Vietnam War protests on his bike. Before his mother died in 1995, she admitted that her late husband was not his biological father. He met his real father - a classical musician - four times before the old man cut off contact. Within a year, von Trier had left his first wife, converted to Catholicism (hitherto believing he was Jewish) and made Breaking the Waves.

It was after seeing this film that Kidman remarked that von Trier was the one director she would most like to work with. It was a remark that led her to a studio in Trolhattan, Sweden, and the iron collar.

Von Trier says: "The good thing is that because you don't pay them very much you know they are doing it because they want to." This also gives him carte blanche to treat actors anyway he wants to, it seems. During the press conference this week for Dogville, Kidman was moved to say that people thought she was mad to work with von Trier.

"Everyone said to me, 'Why are you going off to do this thing with this cruel director?'" she said. "When I arrived in Sweden, the first week was very tricky. At first I cried because he would chastise me. He could be incredibly tough, but he could also be strangely gentle as well, which is the thing that draws me back to him."

For his own part, von Trier is less complimentary about his exalted star. The tales of his bitter disputes with Bjork on the 2000 Palme d'Or winner Dancer in the Dark are now the stuff of cinema legend, and he is known to have pushed the non-actress Icelander beyond endurance to get the performance he wanted. Clearly he did not have to go to such lengths with Kidman. Or did he?

"The process with Nicole was not as different from Bjork as you might think," von Trier tells me. "Nicole was willing to try things that were out of character. Her agent advised her not to take part

The risk appears to have paid off. At the time of writing, Dogville is a sure-fire winner of this year's Palme d'Or and Kidman is well positioned for Best Actress prize. Given the fact that the film has been perceived here as viciously anti-American (an accusation heatedly denied by the director), the risk to Kidman's career in the States is compounded by Hollywood's jaundiced view of the film.

But perhaps the most intriguing aspect of their collaboration is the correlation (forgive my lapsing into the realm of amateur psychology) between Kidman's recent personal history and her career choices.

Since the traumatic process of filming Eyes Wide Shut with her then husband Tom Cruise for the notoriously demanding director Stanley Kubrick, Kidman has shown recklessness in her roles.

She went from the relatively conventional spook movie The Others to the high-risk, low-budget movie Birthday Girl, then from the flamboyant musical of Moulin Rouge to the dark, psychological waters of Virginia Woolf in The Hours.

With Anthony Minghella's American Civil War picture, Cold Mountain, and Robert Benton's The Human Stain just wrapped, she is putting the finishing touches to Jonathan Glazer's Birth before going straight into Mr and Mrs Smith with Brad Pitt. This workload is not just the ambition of an actress who knows her time in the sun is limited but also indicative of a person who is using activity to keep the real world at bay.

With a collapsed marriage to Hollywood's greatest male star and rumours - strenuously denied - of an affair with one of Britain's international actors, Kidman is attempting to thwart the media intrusion into her life by delving inside herself to produce performances that will either expose or numb her feelings through work, as opposed to the more banal process of interviews.

Kidman says: "I like the isolation of working. I want to put my weight behind directors who interest me, though I don't have that much power, actually."

Whatever power she may have to get films made, she has surrendered to von Trier who forced her in public to swear she would make the next two films in his trilogy.

It was like watching Svengali manipulating Trilby to do his bidding. Beneath the good-natured bantering there lurked something more tenacious - an invisible steel hawser that links a Hollywood star prepared to sacrifice herself on the altar of creativity and a visionary priest-like director who is holding a ritual dagger above her.

"Yes," muses von Trier with an inscrutable smile, "I think I did a good job with Nicole."