It's fair to say that Viggo Mortensen has mixed feelings about his Oscar nomination, and the awards season in general. "I don't know why I was included this time. I certainly haven't gone out and done the campaign that one is supposed to do," he says, in so diffident a mumble I have to lean to catch his words. "If you're lucky, you get a lottery ticket, then you have to go find some nice clothes, wash up and go, and it's very flattering, and my mother is very happy about it. But like most people - unless they're very practised at it or have no warm blood at all in their veins - I feel a little apprehensive about the red carpet. It's always a bit bewildering when people are taking pictures and asking questions before the ceremony. I'm not very good at giving quick answers, and that's what they want. So I end up saying half a sentence, and then they go, 'OK, thank you!' and I'm thinking, 'WHAT did I just say?'"

The whole business, he supposes, is just a teensy bit ludicrous. Mortensen doesn't want to sound unappreciative about his inclusion on the best actor Oscar shortlist for his work in David Cronenberg's Eastern Promises, but a) he is indignant that his director has failed to score any nominations, and b) he is under no illusions about winning. Indeed, if ever there's a bad year to be up for best actor, this is it, with Daniel Day-Lewis inexorably locked on to target. "I'm realistic," Mortensen says. "I know I'm the odds-on favourite to remain seated throughout the ceremony."

Mortensen, 49, arrived in London from Washington state yesterday. He comes down from his room jetlagged, barefoot and wet-haired, smelling quite deliciously of soap, and makes short work of the two double espressos that have been cooling on the table waiting for him. A slight, thoughtful presence, he's bearded in preparation for his role in The Road, an adaptation of Cormac McCarthy's post-apocalyptic novel, co-starring Charlize Theron; he has just completed Ed Harris's western Appaloosa, and Good, from the CP Taylor play set in 1930s Germany.

Mortensen was 43 when he landed the role of Aragorn in The Lord of the Rings, ending a long stint on the supporting actors' bench (The Indian Runner, Carlito's Way, Portrait of a Lady), but his choices since then - including a previous film with Cronenberg, A History of Violence - have shown a taste for the unexpected and small-scale. He likes the freedom that comes with limited budgets, where the directors tend to be "left more in peace" by producers; he also likes scripts that take him somewhere new. "I don't have a set plan. I like to learn things. That's the main thing I like about my job."

Divorced and with a son at college, Mortensen is famous for immersing himself in his characters' lives long before he steps on set. This is along with an engaged political interest - he backed the shortlived presidential campaign of progressive Democrat Dennis Kucinich, and will now settle for Obama, though he sounds less than rapturous: "Both he and Clinton have so many corporate ties that are unsettling to me. Perhaps he has fewer. And the movement that's behind him may plague his conscience so much that I think he'll get us out of Iraq sooner than Hillary Clinton will. So, that's our best chance." It means that he's viewed by the industry with respect and bafflement; certainly his young LOTR colleagues talked about him as some sort of visionary or seer.

"This basic thing I always do: what happened between the character's birth, and page one of the script? Anything that's not in the story, I'll fill in the blanks." This has always been Mortensen's approach, even for rinky-dink starter roles such as his brief appearance in Witness, where he appears in straw hat and shirtsleeves for the famous barn-building scene. It made the long haul of fruitless auditioning particularly draining - he got this close to the Christopher Lambert role in Greystoke, to the Willem Dafoe role in Platoon - but at least he came away better informed. For Eastern Promises, in which he plays Nikolai, implacable fixer for a London-based family of Russian mobsters, his research informed not just his performance but the plot itself. The tattoos that play such a key role, for instance, only found their way into the script after he read a book on the subject. Just before shooting, he went backpacking, solo, around Moscow, St Petersburg and the Urals. (Languages are not a problem. Born to a Danish farmer father and an American mother, he grew up in Venezuela, Argentina, New York state and Denmark. There's a UN patch on his sweatshirt and if you ask him where he's based, he says, hippyishly, "Planet Earth, mostly.") The travelling, he says, informed more than his performance. "It changed the way I read Russian literature, poetry, history ... it changed the way I felt about things I'd read before. I like what I learn in preparation. That's the main thing."

His character in Good, a German professor, finds refuge in music, and while filming in Budapest, Mortensen started hankering after a piano, though he'd never played before. An old piano tuner loaned him an upright, "and I asked the hotel if we could bring it in the back door if I promised to put pieces of wood under it or something ... So we put it in the room and I started playing a little bit, and I played it more and more, to the point where every night, rather than looking at the script, I'd just play the piano while thinking about tomorrow's work, and it was really helpful." Nothing goes to waste. As we're finishing up, he hands over a CD, entitled Time Waits for Everyone. Composition, performance, production, cover photography: he did the lot, then released it via his own company, established with the LOTR cash dump. (It's minimalist, melancholy, entirely listenable and owes a certain debt to Satie.)

Mortensen, then, is a bit of a polymath. He paints, writes poetry and has exhibitions of his photographs lined up in Iceland and Denmark later this year. "I can remember being seven or eight and waking up in the morning and going, shit, I'm going to die. Not because I was afraid of dying, but there was this sense - I still have this once in a while - that there's so much to see and do. It made me want to seize the day, whatever you want to call it. Pay attention." He only began to think about acting after college (he studied government and Spanish and - a characteristic flourish - embarrassed his parents by boycotting graduation robes in protest about the manufacturers' poor labour practices) while living in Denmark, delivering flour to bakeries and selling flowers on street corners.

Growing up, he hadn't been exposed to much cinema, so he educated himself: "Older European movies, Japanese movies. In particular I remember Meryl Streep in The Deer Hunter. I saw a really poor print of Maria Falconetti in Carl Dreyer's The Passion of Joan of Arc: she's amazing. Bergman's movies, Autumn Sonata. Bresson. Jessica Lange in Frances. What I saw was a way of doing the job of make-believe that was more than just entertainment. And when it really was well done, as well as appreciating it as literature or art, something else happened, which was that I started wondering: what's the trick? I started being curious about how it worked, how I was moved to really believe what I was seeing, if it was done well." In New York, he joined a drama workshop, was picked up by an agent and began the long slog. "I started getting close to getting jobs - repeatedly. The first couple of years I did screen-tests like Greystoke maybe twentysomething times. I'd do my best, but never got one."

So why did he stick with the acting? "On balance, even though it was mostly disappointments and frustrations and embarrassments, there was something about it that I liked, and still like." Film reconciles the sprawl of his interests: philosophy, history, psychology, photography, music. Sure, it's an odd business, with plenty of pitfalls, but he thinks he has a pretty healthy take on it. "But then," he says, "if I had a really healthy attitude, I wouldn't be in the industry at all. So I must be somewhat contaminated."

· Eastern Promises is released on DVD on February 25