By David Gritten Last Updated: 5:02PM BST 16 Apr 2009  Viggo Mortensen's starring role in his new film Good is being hailed by its distributors as an extraordinary change of pace – a fact that makes him sigh resignedly.  One can see why. In Good, adapted from the acclaimed 1981 stage play by C P Taylor, Mortensen plays John Halder, a mild-mannered German literature professor in the Thirties who writes a novel that advocates compassionate euthanasia. The book is seized on eagerly by senior figures in the Third Reich, who use it for propaganda purposes, and Halder is flattered by the attention the Nazis bestow upon him. Soon he is corrupted by their blandishments, and starts to lose his moral compass.  Certainly it's a long way from his dashing, almost swashbuckling turn as the sword-wielding horseman Aragorn in the Lord of the Rings trilogy. But it's not as if Mortensen has never played complex roles before.  His last two films for David Cronenberg prove the point. In A History of Violence, he was a decent, Midwestern family man who had a dark, hitherto unsuspected past. In Eastern Promises, set in London, he played Nikolai Luzhin, a ruthless tattooed Russian gangster who again was not what he seemed.  "I think on a surface level people are surprised to see me playing such a passive role in Good," Mortensen concedes. "But Halder's passivity only lasts a certain amount of time. Still, it's true, in this film I don't pull out a gun and shoot people." He sighs again: "People like to pigeonhole you. It's easier."  The film is unusual, says Mortensen, "because Holocaust-related stories generally have some sort of catharsis – a heroic or tragic conclusion, with something big or extraordinary happening at the end. But what that does is to allow you a safe distance. This movie doesn't allow you that safe place. This story is not just about Germans. It's about when people don't pay attention to what they know to be true. Anyone can identify with those moments in life where circumstances or people inform us that we've strayed from the path of our better nature and intentions. We know what that's like, and we resist it – so as not to feel like we're bad people. But finally Halder is confronted with the decisions and compromises he has made."  Some of the film's best scenes are between Mortensen as Halder and British actor Jason Isaacs as his raffish Jewish friend Maurice, who can see clearly what the Nazis are up to, and how they have seduced Halder into their schemes.  Mortensen's relationship with Good goes back a long way: "In 1981, I was sent over here to London from New York to do a screen test for a movie. I saw Alan Howard play Halder on stage, at the Donmar Theatre, and I remember how brilliant he was. And then 25 years later I was given the script, recognised the story and thought to myself: 'This was meant to be.' I would never have imagined I'd play this role."  Mortensen, 50, is a man of many parts, in more than one sense. He is a poet, photographer and painter, and in New York, where he lives, he runs a small publishing house. He is the son of a Danish father and an American mother, and he is fluent in English, Spanish and Danish.  He lived in Argentina for part of his childhood and is a fervent supporter of the Buenos Aires football team San Lorenzo. Superstitiously, he always wears something about his person in the club's colours; today a thin woven cord in red and blue hangs around his wrist. These many interests give his life some perspective, he says. "I think maybe because I do other things and they mean as much to me as movie acting, it takes the onus off me. It's not the end of the world if I can't get a film job, or if a movie doesn't turn out well – even though I don't like it when that happens. There are other things I enjoy doing, and I involve myself in them. It's not like if I'm out of work I'm thinking, 'What am I going to do? I'd better take a bunch of drugs.' "  Mortensen's next role is a high-profile one: he stars in the film adaptation of Cormac McCarthy's Pulitzer Prize-winning novel The Road, a story of an unnamed father and son voyaging through a post-apocalyptic landscape.  "The Road is about that fear that all parents can have," says Mortensen, who has a 21-year-old. "What's going to happen to your child if you're not around? It takes those concerns to an extreme. In the film, without me the boy has no food, no shelter, no resources at all.  "Like Good, it's a universal story." He smiles. "And that's the best kind."  'Good' (15) is released on Friday
Viggo Mortensen returns in a risky role as a professor corrupted by the Nazis    in 'Good'.