Viggo Mortensen would like you to know that his penis is far bigger in real life than it appears in the nude brawling scene in Eastern Promises. Just in case you were wondering. He hasn't said this out loud of course; he's far too decorous. Instead, we've done him the favour of inferring it from the following exchange:

LWLies: Is it weird to know that so many people have seen you naked?

Mortensen: "Erm... No, I knew going in that it was... I guess when you stop to think about how many people screen grab or selectively look, it's like, well, it's not the most flattering position, or something..."

He squirms a bit at this point.

Mortensen: "I don't know, but what are you gonna do? You've got to play the scene."

Mortensen, it is clear, is an actor with an admirable sense of duty. On this crisp winter day, he is sitting in a plush suite in a posh London hotel on the latest leg of the publicity your for his new film Good. A little dwarfed by the ridiculously plump sofa cushions, he is less imposing than you'd expect Aragorn, King of Men to be. His hair is mostly grey (Mortensen turned 50 last October) and has been allowed to grow over the tops of his ears. He's wearing dad jeans with a navy fleece and he's taken his shoes off. He seems tired, but then so would you if you'd recently flown through three continents in less than a week, and tiredness notwithstanding, he is thoughtful, engaged and unfailingly polite.

Born in New York and raised in Argentina, Venezuela and Denmark, Mortensen spent time as a stage actor on the East Coast before moving to Los Angeles and beginning his screen career with the role of an Amish farmer in 1985 thriller Witness. A series of quality supporting roles followed - alongside Al Pacino in Carlito's Way (1993), in Tony Scott's Crimson Tide (1995), and in Gus Van Sant's Psycho (1998). We'll call this the pre-Lord of the Rings era, because after Peter Jackson's trilogy, everything changed. Along with the other principle cast-members, Mortensen was rocketed into the stratosphere of leading men, just in time for his forty-third birthday.

This comparatively late arrival has had its advantages, he says. "I learned more about how fleeting the sort of attention that sometimes comes your way can be, but mainly just how to be professional and help contribute to telling a story in the movies." Mortensen uses the phrase 'telling a story' several times over the course of our interview, where other actors might say 'playing a role' or 'getting inside a character'. It's revealing because it illustrates how he wishes to be seen; not as a virtuoso performer, but as part of a team, doing a job.

He is, however, rather an important member of that team. Mortensen is the kind of name that gets studio execs on the phone, puts scripts at the top of option piles and turns red lights to green. If the relatively small number of roles he's taken on recently is anything to go by, this is a responsibility he takes seriously. His two collaborations with David Cronenberg. A History of Violence (2005) and Eastern Promises (2007), were particularly well received and Mortensen has an idea why: "We trust each other and we know that we're both in it for the same reason. We want to tell the story really well... It's not often that you find you're on the same wavelength with another artist."

Hollywood casting agents take note: if you want Viggo Mortensen in your movie, you better have a great director and a damn good script. Or, failing that, at least make sure there are some horses. Mortensen, an accomplished rider and the author of a photography book simply titled The Horse is Good, has an unfortunate weakness for the four-legged beauties, which explains the tedious one-man-and-his-mare epic, Hidalgo (2004).

Horsey movies aside, Mortensen's post-LOTR roles share a unifying theme. As Aragorn, he played an archetypal man of action and every role since has seemed like a variation on that theme. Tom Stall, defender of the American home, Nikolai the Russian tough guy, a swashbuckling solder in the Spanish language Alatriste (2006) and a manfully taciturn cowboy in last year's Appaloosa. His character in upcoming Cormac McCarthy adaptation, The Road is simply called 'The Man'. And then there's Good.

Mortensen plays John Halder, a university professor in 1930s Germany, and a man so constitutionally averse to taking action that he gradually becomes complicit in the Nazi regime. If you've seen Valkyrie, Defiance or any of the big, blustering World War II movies out lately, then Good's pacing may surprise. "There isn't a heroic gesture or a big tragic moment," says Mortensen, moving to the edge of the sofa as he explains. "I don't go down in a hail of bullets or save everyone or whatever, and most movies about that period, they give you that, right? This one doesn't. You're not let off the hook as an audience member."

Conversely, Mortensen conducts his own life with the kind of straightforward integrity which would be considered embarrassingly unconvincing in a modern movie protagonist. He maintains a dignified cloud of mystery around his personal life ("The people who have a lot of trouble with the press either consciously or unconsciously invite it.") and he takes action for the causes he believes in. In 2004, film critic Michael Medved accused Mortensen of "ill-timed political posturing" and "pacifist preening", but Mortensen remains uncowed. "I don't just say anything. I try to stay informed as a citizen in the world, but I've always felt that when you know something's not right and that you could say something, to not do so is the same as lying. I'm not gonna shut up just 'cos someone tells me to shut up."

You'll notice Mortensen has segued nicely back to the theme of the film he's promoting. He's clever like that. If you were a film director, he's exactly the kind of actor you'd want talking up your movie: on-message, but not in a bland, PR-trained sort of way. He's passionate, informed and willing to answer a question like 'Is it weird to know that so many people have seen you naked?' without the slightest murmur of complaint.