There was a time, when he was making pictures such as Videodrome and The Fly in the 1990s, that director David Cronenberg might have had the Russian mobster in his new film Eastern Promises turn into a giant balalaika-playing cricket, just to freak people out.

As it is, the metamorphosis in the film belongs mostly to Viggo Mortensen, the actor who plays Nikolai and, until now, was best known as the star of the Lord of the Rings movies.

Whatever else audiences or critics may make of Eastern Promises, Mortensen will be transformed from a Hollywood hunk who hasn't done a five-minute fight scene totally naked to one who has, totally.

That is no small thing. (By which we mean to imply absolutely nothing about Mortensen's physiognomy). The inherent sexism of Hollywood has made female nudity commonplace. Mortensen's co-star in Eastern Promises, Naomi Watts, has frequently taken off her clothes, although not in this picture. Yet with the exception of Harvey Keitel and Ewan McGregor, who can't seem to keep their pants on in movies, male stars have remained as modest as the vicar's wife.

Because the script by Steve Knight (Dirty Pretty Things) is a deadly serious take on what Cronenberg calls "criminal globalization," Mortensen's fleshy fight scene sprang not from prurience, but from a desire to make Nikolai's vulnerability starkly apparent. He is meeting with another immigrant mobster in a London Turkish bath when he is set upon by men with knives. And clothes!

"The word 'naked' was not in the script," Cronenberg says. "Once we started to talk about it, Viggo said, 'Well, it's obvious I've got to play this naked,' and I said, 'Yeah, great.' And that was it. No big deal."

No, big deal.

During a recent visit to San Francisco to promote the film, Mortensen and Cronenberg spent about half of every interview discussing the scene's choreography, which could easily have been shot and edited to reduce the star's exposure. "I didn't want it to be like a Bourne movie, with a lot of quick cutting, where you don't really see anything," Cronenberg says. "I want to see everything. I'm very body conscious anyway, and I wanted it to be very difficult, painful work."

"This is a side of Nikolai that you hadn't seen fully," Mortensen says without evident irony. "He's a survivor, he's determined, and he's going to go out of his way to do whatever he thinks is the right thing to do."

He could just as easily be referring to Cronenberg, for whom the actor has a towering respect that, true to form, he makes no attempt to hide. The two teamed up for the first time in 2005 on A History of Violence, in which Mortensen's character is almost the exact inverse of Nikolai, whose progression is into a world of gangsters, rather than out of it.

A History of Violence was a movie more often esteemed by critics than seen by audiences, a price that Cronenberg has paid - sometimes grudgingly - for pursuing a harrowing artistic vision, in which humans are transformed into cockroaches, flies and, once, even a VCR. By making two pictures in a row that are more naturalistic, Cronenberg appears to have made a transformation of his own, into a more mainstream director.

"I make two or three thousand decisions a day that are unique to me," he says. "I don't have to worry about the movie being me, or not me. I don't worry about putting my touch on it, or if it's thematically connected with something else I've done. Those things simply are not creatively useful at all."

Mortensen has benefited from Cronenberg's direction in a pair of performances so deftly underplayed that it's sometimes hard to tell he's acting. "You need a director who pays attention to the little reactions in the interplay between the characters," Mortensen says, "I like his touch. He respects the audience, and leaves a lot of things unanswered.

"He presents the situation, he presents characters just like you see people on the street. You might stop and talk to someone, fall in love with someone, get into an argument with someone. But in the end you don't get to know them fully; you get to know them in that situation and that's it. That's what life is like, and to me that's why his movies, no matter how strange they might seem, are like real life to me. They are disturbing and thought-provoking and uncomfortable, like real life can be if you pay any kind of attention."

Though Eastern Promises is a serious film from a serious filmmaker, Cronenberg doesn't sneer at questions about Mortensen's fight scene in the Turkish bath. He wants people to see the movie, and if that's the hook that draws them in, he doesn't mind. "We certainly didn't think about, 'This is the shower scene that everybody's going to talk about and come to see,'" he says.

But he also doesn't spoon-feed the audience, at times making it difficult to figure out what's going on, or how the characters all fit together. "It bothers me to think that I'm making a movie for stupid people," he says. "And I refuse to accept that. That's basically all there is to it."