Viggo Mortensen has starred in a $1-billion-grossing hit and worked with Demi Moore and Buckethead. He speaks seven languages, runs his own publishing house and names Argentina's San Lorenzo de Almagro as his favorite soccer club.

''The thing about San Lorenzo is that they're kind of like a bad luck team,'' he says during a recent interview. ''They never have star players; they're the eternal underdog.''

Mortensen was something of an eternal underdog himself, at least until the Lord of the Rings trilogy crowned him a box-office king. But instead of looking to consolidate his mainstream appeal, the actor went straight back to the margins, signing on for a Spanish-language film called Alatriste and scooping up the tricky role of a mob assassin-turned-family man in Cronenberg's A History of Violence.

Working with Cronenberg was such a great experience for the actor that he was eager for a rematch. Mortensen agreed to appear in Eastern Promises, the director's latest film, without even reading the script.

''David called me one day and said, 'I think I'm going to do this thing about the Russian underworld. How would you like to do that?' I said, 'All right,''' recalls the actor. ''It was an easy decision to make. As strange as people might think David is, he's also a compassionate person and ultimately very respectful of the people he works with.''

In the film Mortensen plays a taciturn, heavily tattooed chauffeur who finds himself and his Russian-mobster boss (Armin Muehler-Stahl) drawn into a mystery surrounding the death of a pregnant teenager. A midwife (Naomi Watts) suspects foul play and vows to figure out the circumstances behind the young woman's demise.

Before production began, Mortensen flew alone to Russia on his own dime in hopes of picking up authentic slang words and soaking up the culture. He refused to take a translator along for fear a language expert might ''filter'' his experience.

''I went to Moscow and St. Petersburg and just walked around,'' says Mortensen, 48. ''I used the subways. I took buses into the country. I went to villages. I finally got a hold of a car and then I really went all over the place.''

Meeting with members of the Russian mob was never a dangerous proposition, alleges Mortensen. ''I knew that some of the people I spoke with were not folks I'd normally hang out with,'' he says. ''But there was this truce between us because they realized that I was out for authenticity.

''Russian characters in American and European movies are always pretty clichéd. There's not much of an effort made to get things right. I wanted to change that.''

Mortensen's dedication to authenticity extended to a bathhouse sequence in which he takes on a couple of bad guys clad in nothing but his birthday suit.

Cronenberg says it took all of two seconds for the actor to decide to drop trou. ''Basically, as we were choreographing the scene and working it out with the stunt coordinator, Viggo said, 'Well, it's obvious I have to play this naked.' And I said, 'Yes, naked.' That was it. That was pretty much our whole discussion on the matter.''

Mortensen is similarly nonchalant about the sequence. ''Look, I trust David,'' he shrugs. ''And I think my nakedness adds to the scene. It looks uncomfortable because it is uncomfortable being naked and being thrown to the ground and forced to roll around and smash into stuff.''

Cronenberg believes Mortensen's nudity gives the sequence an almost Hitchcockian level of suspense. ''The nudity has a dramatic importance,'' says the filmmaker. ''Most people associate nudity with eroticism and sex. But when you think of the shower scene in Psycho, it's really about vulnerability. It's the same idea here: Viggo is wet and naked and defenseless and suddenly somebody wants to kill him with a knife. That is quite scary.''