`Come on, you're obviously a man of average intelligence . . ."

"Only average?"

Willem Dafoe's laugh, like gravel in a cement mixer, resonates through the Hilton Hotel's trendy bar.

He'd been fighting shy of providing any deep analysis of the Wooster Group's production of The Emperor Jones by Eugene O'Neill, which has brought him to Belfast. Interview after interview briefly mentions Dafoe's 20-year career with the group as his "important work" and then rushes on to discuss his latest movie role; I had promised to focus on the theatre. But Dafoe is obviously uncomfortable with the fact that his star quality deflects attention away from the ensemble nature of the theatre's work, and particularly, the directorial role of his partner, Elizabeth LeCompte.

Born in Wisconsin 45 years ago, he first toured with the Milwaukee-based Theatre X, but in 1977 fetched up in New York, and sought to join the Wooster Group, because "Elizabeth LeCompte was doing the best theatre in the world, as far as I was concerned. I wasn't thinking about tomorrow, I wasn't much more than 20 years old. All I cared about was what I found thrilling. It's very weird to have a group of people who own a space and make theatrical work. And their theatrical language was very fluid, very visual, anti-illusionistic and very abstract. It reminded me of cartoons." Apparently, there was no part for Dafoe when he arrived, and LeCompte responded to his presence with the words: "I don't know who the f---k this guy is, but get him out of my house." Over 20 years later, they are still together and have a teenage son, Jack. My challenge to Dafoe's intelligence came after his disclaimer: "I really think this is where you're stuck with the stupid celebrity interview, for the painfully obvious reason, that Elizabeth is the real star. She's the brains and the passion and the fierceness behind the work." Later on in the interview, he says: "This is so hard for me. I am oddly . . . Certainly I contribute. But I'm more passive. Of course, I'm a theatre-maker, but I'm an agent of Liz's passions and curiosities."

He doesn't make the crude qualitative distinction between theatre and cinema work with which he's been credited: "With a movie, everything's so fragmented. You don't stay with things for very long. With the theatre, you keep on revisiting the same ground. It's more like life, you wake up and you have to work out how to negotiate the day. In cinema, you're dealing with your initial impulses - sometimes they're the truest impulses - but it doesn't have the same mystery. I have a performance tonight and I have to address myself to the making of the performance and why I do this. In theatre you have to keep the music alive. In film - it's the newness of it all which feeds it."

The Woosters' theatre, with its use of masks and stylised Oriental forms of drama, seems further from cinema than theatre often is. The Noh and Kabuki influences remain unexplained - when asked about them at a "meet-and-greet" session on Sunday at the Lyric, Elizabeth LeCompte explained their presence in the work with the words: "We watch a lot of television."

In The Emperor Jones, for the part of Brutus Jones's cockney accomplice, Smithers, Dafoe wears a mask of whiteface make-up: "I think you can reveal yourself through a mask so much better. Then the transformation is so much easier. You imagination gets refreshed by that different orientation. It allows you to play and imagine more, and you're not protecting your own persona."

Surely this is a level of distance he can't have when a movie camera fastens on his expressive face? He counters that there are many types of masks: "In Tom and Viv, it was the accent, in Platoon and in The Last Temptation of Christ I had other personas."

Coming from such an intense, ensemble tradition of theatre - surely he chooses his film roles with extreme care (with a rumoured asking price of $20 million a film, he can afford to). Dafoe is refreshingly lacking in preciousness about his work - in fact it is strange that someone with such a reputation for playing weirdoes is obviously one of those people who can't help but try to charm: "I don't go comatose when I hear the word `career'," rattles the gravel. "My aspirations are complicated and very mixed. I have to weigh up each offer against the (Wooster) group's needs - sometimes selfishly, sometimes less than selfishly."

He didn't set out to branch out from theatre into movies; it just happened. "Someone saw me in a group piece we did and asked me to be in a movie. I enjoyed it. Then there was the first studio movie, Streets of Fire (1984) and then the first major role in To Live And Die in L.A. (1985)." Was celebrity status a shock? "In retrospect, yes. It's almost like I didn't realise what a big deal it was at the time. My son's babysitter called me one day at seven in the morning and told me I'd been nominated for an Academy Award for Platoon."

In fact, Dafoe's unforgettable performance as the sergeant betrayed by his own troops, and left to die of his wounds in the jungle in Oliver Stone's Vietnam epic (1986) ensured that he would never again be anything less than a celebrity in his home country. Even in the dark corner of this Belfast bar, the boy at the next table is pushing out his fragile paper beer mat for an autograph.

His next film role, in Mark peploe's adaptation of Joseph Conrad's Victory, which opens in the UK on Friday, sees him revisiting the colonial tensions of The Emperor Jones, as well as the steamy jungles of Platoon. Shot in Indonesia, Malaysia and Berlin, the film, says Dafoe, "is very much about a colonial world where all the world is converging in these outposts. My character, Heist, meets a white slave girl, and she begs him to get her out of the situation she's in. It's like, a moral dilemma. The guy says: `OK, I'll take you with me to this island where he really wanted to be alone at. She falls in love with him because of his generosity - she represents his coming back to the world. This sets up a chain reaction . . . "

"It upsets the colonial order?"

"Some people want to mess it up and it has tragic consequences."

He doesn't want to talk too much about his next project, Dust, by the Macedonian director, Manckewski, who made Before the Rain: "It's about an uprising against the Turks at the turn of the century. I play a US gun-slinger - a mercenary cowboy. I'm learning to ride."

"That must be fun."

"It's a lot of fun."

Suddenly, as we prepare to leave, he is in spate, and proving he can cross that "average intelligence" mark any day of the week: "As an actor, more than anything else, I'm really trying to get to grips with what is. Like a musician, who's really hearing the music. It's a mindfulness and an awareness which allows you to have grace in the actions you do. There's the pretending and the doing. The pretending is the frame and the doing is committing yourself in a full way to really doing the thing."

The Emperor Jones plays tonight at the Waterfront Studio, Belfast.