In his latest film, David Cronenberg's Eastern Promises, the 48-year-old Lord of the Rings star acts out a brutal, four-minute fight sequence in the confines of a steamy Turkish bath while entirely naked. During the bruising sequence, he's forced to show off his manhood in every conceivable sense.

"I'm just glad it was shot in two days and not six as it was initially written," he laughs. "Honestly, though, it took me all of about eight seconds to decide whether to do the scene naked."

The scene comes midway through the movie, in which Mortensen play Nikolai, a Russian mob member working for the Vory V Zakone criminal brotherhood, a man whose carefully maintained existence is thrown into confusion when he meets a young midwife, played by Naomi Watts.

The noirish movie, set in the London underworld, contains a number of unpleasant scenes, but the bloody fight sequence is by far the most protracted, with Cronenberg leaving the camera lingering; there are no quick edits to save the actor's dignity, or the audience members' nerves.

"It was slippery, painful, embarrassing," says Mortensen. "You see what you see. I always knew the scene should be as realistic as the rest of the movie, so I couldn't feasibly keep the towel on.

Plus, we shouldn't be trying to hide things or pretty it up in any way. I knew at times it would be awkward and vulnerable. It would also be painful because I couldn't wear any pads to protect me. All I was wearing was a bunch of tattoos."

Indeed, Mortensen's body is covered in ink. Tattoos serve an important purpose in the world of the Russian criminal, and with both actor and director slaves to detail, each piece of body art was carefully chosen and applied.

Cronenberg says that during pre-production, Mortensen sent him a book on Russian criminal tattoos, which was filled with explanations of their meaning, as well as countless pictures. Several of these were then transferred to the movie, standing, says the director, as "intense metaphor and symbol". Mortensen's character sports 43 tattoos in all.

"I really believe in mining as much meticulous detail as possible," he says. "It's something that I tried to do when I went travelling in Russia. I wanted to get a handle on the people and language before we came to London. Getting that level of detail, to really create this world, was of paramount importance, and I think the world we create is quite true to life.

"During my travels in Russia, I met all these nefarious people. I travelled everywhere alone, without a guide, and I met people who had been in the Russian underworld, or who had spent time in prison."

Mortensen notes that, while his fight sequence seemed to get most of the attention when the film premièred in Toronto, the film notches up fewer deaths than many of its bigger-budget contemporaries. "In terms of being violent, yes, it is, but the body count in this movie is far less than in The Departed or the Bourne movies," he says.

"People will like to say that Eastern Promises is brutal, but the only reason they say that is because the scenes stick with them. They are realistic. They are in-your-face and you see the consequences. It's not a bunch of quick editing cuts. The same is true of A History of Violence."

Like Eastern Promises, A History of Violence - the first collaboration between Mortensen and Cronenberg - showcased the director's ability to blend different elements of genre, and to extract multi-faceted performances from his leading man.

The film was released in 2005, two years after the final chapter in the Lord of the Rings saga, in which Mortensen played the noble king Aragorn; after the disappointment of his 2004 equine adventure, Hidalgo, it helped to cement the actor's reputation as a leading man.

Born in Manhattan to a Danish father and American mother, Mortensen has enjoyed a varied career. He landed a string of coveted roles straight out of college, appearing in Jonathan Demme's Swing Shift and Woody Allen's much-lauded The Purple Rose of Cairo. [Shame he never made it to the final cut! - Articles Ed.]

As his career picked up, he shone in Sean Penn's muscular directorial debut, The Indian Runner, and was excellent during his brief wheelchair-bound performance in Brian De Palma's crime movie, Carlito's Way. His big break though came courtesy of Tolkien.

"If Aragorn hadn't became well known," he smiles, "and I hadn't got that visibility in The Lord of the Rings, there's no way that any production company would let me play Nikolai in Eastern Promises or Tom Stall in A History of Violence, or star in [Spanish movie] Alatriste. I simply wouldn't have got that chance.

"And, truthfully, I was never worried that people would only ever see me as Aragorn. In fact, I almost didn't even join that film, because they'd already started shooting and had been preparing for months."

The original actor cast as Aragorn, Stuart Townsend, parted company with the production, leaving the door open for Mortensen. "I hadn't read the books or anything," he continues. "But when I was finally offered the chance to play Strider/Aragorn, I became curious because I thought it might be something that I could regret not doing."

So far, Mortensen has no regrets, building a credible body of work that highlights his redoubtable talents. He is currently in talks to star in The Road, an adaptation of novelist Cormac McCarthy's Pulitzer Prize-winning vision of a post-apocalyptic America, to be directed by The Proposition director John Hillcoat.

"Who knows what I'll do in the future," says Mortensen with a smile (although a copy of The Road is peeping out of his agent's handbag as she comes to collect him). "But I enjoy working with iconoclasts, like Gus van Sant. He is an iconoclast, as is David Cronenberg.

"One thing is for sure, though, I'll definitely work with David again. And if he wants me to fight naked, so be it!"