A painter, poet, jazz musician, and political activist, Viggo Mortensen is a mass of complicated, sometimes conflicting energies and interests. He's as macho and swarthy as they come, but with a contemplative thirst for truth. He's shy, but a bit of a motormouth (and can run on in at least six different languages). Mortensen is a matinee idol with a philosopher's soul — Jean-Jacques Rousseau trapped in the body of Rudolph Valentino.

When I interviewed him last month during his stop in San Francisco to promote the David Cronenberg–directed thriller Eastern Promises, it became clear that the strong-yet-delicate thing isn't just a clever shtick. Looking tan and lean and sporting an impressive 'stache, he was soft-spoken and friendly. It didn't hurt that he came bearing gifts — before I even sat down, he placed a shrink-wrapped copy of Exene Cervenka's book of collage, 666, on the table in front of me. (Mortensen's boutique company, Perceval Press, publishes the book by the artist and X frontwoman, who is not so coincidentally his ex-wife and the mother of his teenage son, Henry.)

What sometimes gets lost in the Viggo-induced swoon is that the man is a fine actor.

Mortensen is often the best thing in his movies, though in the past that sometimes wasn't saying much. After delivering what should have been a star-making performance in Sean Penn's 1991 directorial debut, The Indian Runner, he languished in B-movie hell (American Yakuza) and dud big-budget productions (Boiling Point, Daylight). Peter Jackson might have given him the exposure he was due in the Lord of the Rings trilogy, but it took a weirdo genre director eager to flex more commercial muscles to give him the roles he was born to play: sensitive, soul-searching, primordial beasts.

In Eastern Promises, his latest collaboration with said weirdo director, Mortensen plays Nikolai Luzhin, a driver and all-around henchman for the notorious Russian organized crime syndicate Vory v Zakone. During its making, Mortensen helped literally and figuratively to flesh out the idea for what became a major thematic refrain — the detailed prison tattoo work found all over Nikolai's body. "[Tattoos were] mentioned in the original script in passing," he noted. "But like everything else, I wanted to know what that meant. A friend of mine, Alix Lambert, made a great documentary called The Mark of Cain, where she went into maximum-security prisons in Russia and learned about Russians and Ukrainians and Georgians — men and women — who have identified themselves with these symbols. I learned, among other things, that symbols and text — religious or other — that seem to mean one thing on the surface actually mean something quite different. It's a CV, a résumé, that they have on their bodies."

Mortensen studied Russian for the role and traveled to the country for research. "I checked with people who had backgrounds not dissimilar to the character I was playing. Once they realized I wasn't trying to mock them or wasn't going to do yet another clichéd Russian or be critical of them — I was just trying to get it right — then they were very helpful. So the tattoos were correct."

Mortensen acknowledges that his comfort level with Cronenberg has freed him to do ...