Viggo Mortensen doesn't just play twins in his new noirish thriller, he also took charge of the subtitles. The actor talks about challenging roles – and why he turned The Hobbit down

Viggo Mortensen is softly spoken, clean-shaven and casually dressed in jeans and a T-shirt. If you saw him in a restaurant, you'd smile at him not because you'd think there's a huge movie star, but because he radiates a gentle integrity and, well, niceness. But he's a disconcerting interviewee. The conversation goes like this. I ask question A, expecting answer B. He listens carefully, considers, and gives me answer E, and then we find ourselves on point K, V, or Z.

Luckily, we do keep returning to Everybody Has a Plan, a film that's close to his heart. Although it's the fourth he's done in Spanish, it's his first Argentinian movie. "It was like going home," says the star,...