"Being familiar with Tolkien's story, Henry made a very convincing argument for my accepting the challenge.
"I have a Scandinavian side," he says, "that mindset of orderliness, a need for a logical explanation for things.
I understand the sense of humour and irony that they have.
Yet there's clearly a lot more to the Danish-American, both in the range of his work and his personality.
When I meet him in London, the first impression is full of contrasts.
"We both thought the script was really funny, and tried to push the comedy," adds Fassbender.
"One day we were shooting a scene and Viggo kept nudging these ceramic penises on his desk towards me - until they were surrounding my coffee cup." For many, the prevailing image of Mortensen is of the heroic Aragorn from The Lord Of The Rings, a pretty straightforward chap given that he's surrounded by mythical creatures and is dating an elf.Highlights were the bad-apple brother in Sean Penn's The Indian Runner, and Demi Moore's tough but philosophical instructor in GI Jane (again amusingly moustachioed and atypically literate). I wonder if being in his early forties when that level of fame struck helped keep his feet on the ground."I think it's always good to work as if any particular day of filming might be your last.The stand-out achievements have been those with Cronenberg, especially A History Of Violence, and Eastern Promises, the latter earning him an Oscar nomination. "In some ways he's a different kind of person entirely than Lisandro, but they're similar in that they ask lots of questions, but don't give answers.I think it's great for an artist to do that, it's respectful of the audience; it can be annoying and frustrating, but it gives you something to chew on." An accomplished poet, painter and photographer, he also owns his own publishing company, Percevel Press.With piercing blue eyes, well-groomed hair and in crisply ironed shirt and slacks, the handsome 56-year-old seems surprisingly formal.