It's not gentlemanlike to bring up a man's age. But how the hell did Viggo Mortensen get to be 55? Heck, he's old enough to be my slightly older brother. Of course, Mortensen has the sort of Scandinavian features that wear the years lightly: sharp cheekbones, greenish eyes, chisel chin. So we shouldn't be surprised that he's kept middle age at bay.

The psychological disconnect has more to do with his relatively late arrival as a major force. Mortensen has been acting regularly since the early 1980s. But it wasn't really until Lord of the Rings, a little over a decade ago, that he began properly shaking the earth.

“Many times over the years, right from the start, I’ve had thoughts about the work being embarrassing or frustrating,” he says. “I have regularly considered doing something else. I have always had an uneasiness about it. One of the results is that I keep thinking this hasn’t been going on very long. I am shocked to recall that it started more than 30 years ago.”

All that noted, in his latest film, the fine The Two Faces of January, Mortensen is definitely playing the "older man". Hossein Amini's adaptation of a 1964 Patricia Highsmith novel could hardly be more deeply marinated in that writer's characteristically sour juices. Viggo and Kirsten Dunst appear as an American couple travelling through Europe who, after becoming caught up in a mysterious death, enlist the help of a cynical drifter, played by the suddenly ubiquitous Oscar Isaac.

As in other Highsmith adaptations, such as The Talented Mr Ripley and Strangers on a Train, Amini's gorgeous picture invites us to root for apparently amoral people as they attempt seemingly unforgivable acts.

“A lot of the best movies are like that,” Mortensen says. “They deal with bad guys, thieves and murderers. But you want them to get away from the cops. That’s often the way with film noir, for instance. You know they probably won’t get away with it, but you hope they do, because they are interesting. There’s also a vulnerability about them.”

Highsmith's stories tend to translate well to the screen. Cinema enjoys teasing its congregation with moral conundrums and, of course, the postwar European settings are easy on the eye. Mortensen directs us towards René Clément's earlier adaptation of The Talented Mr Ripley, Plein Soleil (1960).

The new film is, maybe, more like Plein Soleil than any of the others, he says. "But, of course, it looks a little like Anthony Minghella's 1998 version of The Talented Mr Ripley. But that film cost about three times as much. Add in inflation and it cost maybe five times as much. But there is a virtue to a tighter budget. Ours doesn't call attention to itself the way that did. Without that, you can get closer to the story."

Viggo Mortensen thinks deeply about his art. A slow talker, at home to the odd ramble, he comes across as the sort of sober fellow who will only make a joke when absolutely certain the situation demands it. He’s a hard man to concisely assess (a good thing for an actor, one assumes).

Born in New York to an American mom and a Danish dad, he moved to Argentina with his family when he was just a boy. He also spent some time in Venezuela and the old homeland of Denmark. To raise a trivial point, his complicated background must kick up difficulties when, being a serious football fan, he has to decide whom to support in the World Cup.

“I’ll be supporting Argentina,” he says with a hint of guilt.

Would he have supported them even if Denmark had qualified?

“Yeah. I would, to be honest. But they’re not there. They just tied too many games. I hope the US does well, obviously. I wouldn’t mind if Spain won just because nobody thinks they can do it twice in a row.”

That sort of background must be useful to an actor. Every time a child or young person moves he has to learn new conventions. He has to find more masks to pull on.

“I run into a lot of actors who seem to have had itinerant childhoods,” he says. “Maybe that is important. It’s about taking different positions. It’s about taking different points of view. That may be part of it.”

He is about to go off on one of his verbal rambles. These are usually worth heeding.

“When you travel and become comfortable in different places, it’s the best anti-war tool we have. You are far less likely to be convinced by a president, king or prime minister to bomb hell out of somewhere if you have been there and realise those places are full of ordinary people. People on buses, disabled people, school kids. If you think of them as ‘other’, they are that bit easier to hate.”

Mortensen studied Spanish and politics at St Lawrence University in upstate New York and then set off on another series of travels. He sold flowers in Copenhagen. He drove trucks. He tended bars. Then some sort of nagging desire to act finally took over. As Viggo tells it, he was swayed by exposure to a series of classics from European cinema. Watching Dreyer's The Passion of Joan of Arc and Bergman's Autumn Sonata, he became fascinated with the mechanics of acting.

“Yes, I was transported. I began to think: how do they do that? How do they make that so real? Do they think of their mother and father or whatever? How does this work?”

There were a few lean years. But, despite not achieving high-end fame until his 40s, he seems to have worked fairly consistently. You can see him in Peter Weir's Witness, Sean Penn's The Indian Runner, Brian De Palma's Carlito's Way, Jane Campion's The Portrait of a Lady and Gus Van Sant's Psycho.

We cannot, however, escape discussion of the elephant in the room forever. The story goes that he got the part of Aragorn in The Lord of the Rings at the very last moment when our own Stuart Townsend dropped out. Is that myth based on fact?

“I only know what I have been told,” he says. “Stuart was cast and I think he would have been ideal. I think maybe he was a little similar in age to some of the Hobbits – a bit too youthful. I don’t know how it went down, but I heard that Stuart realised that and that it was a mutual decision. He was doing tests where they were trying to make him look older. But they felt they’d always be worrying about that. So, they just went for an older actor.”

It seems hard to believe now, but Mortensen didn’t immediately jump at the role. He was aware that the rest of the cast had a head start on their sword-fighting, canoe training and elvish language lessons. This notion of making three films in one go constituted, at that point, a very considerable gamble. What’s more, he’d never read the books.

“I’d just got back from a long trip after shooting a film,” he remembers. “I said no. I wouldn’t be prepared. My son overheard me talking. He knew the story and he started quizzing me on which role I was playing. He said: ‘That’s Strider! That’s the guy who becomes king. You’ve got to do that.’ Ha ha! I might have decided to do it anyway. I don’t know. But he pushed me towards it.”

Mortensen took a singular approach to his newly minted superstardom. He retired to his second home in (why not?) Idaho and set to work on poetry, photography and music projects. He has published well over a dozen books and released nearly as many albums. When he came back to cinema, he steered away from blaring blockbusters and formed a durable partnership with David Cronenberg on A History of Violence, Eastern Promises (for which he was Oscar nominated) and A Dangerous Method.

Later this month, Lisandro Alonso's Jauja, in which he stars and which he co-produced, will screen in the Un Certain Regard section at Cannes. So it doesn't look as if there were too many downsides to his belated success in the Tolkien trilogy.

“Well, it was great that people liked the work,” he says. “But it was a bit unsettling because it was everywhere. Until you experience that you don’t know what it’s like. But you have to be aware that it eventually is going to go away. The thing is to not become too enamoured with it. I don’t have any problems with that fame declining.”

After all, as Viggo said earlier, he still feels like he’s only at the beginning.

“I’m still figuring it out. I’m still figuring it out.”