The role of the warrior-king Aragorn in The Lord of the Rings and the 18 months of shooting were big challenges for Viggo Mortensen - but, as he told Ryan Gilbey, he found plenty to identify with in the character.

Viggo Mortensen is a serious and impassioned actor whose apparent severity extends to his Nordic features: he has hard blue eyes, and a pair of cheekbones that could double as bookshelves. He is also an accomplished artist, photographer and poet - in short, not at all the sort of person that you would expect to find represented as a 3-inch high action figure in a Burger King children's meal. But such is the price of appearing in a $300kerzillion epic blockbuster trilogy: Viggo Mortensen, along with the other stars of the Lord of the Rings films, which kick off this month with The Fellowship of the Ring and continue over the next two Christmases, will have to get used to seeing himself splashed across billboards. Yesterday, I saw him on the side of the 357 bus to Chingford Hatch. When I ask him if he has encountered his own action figure yet, his sniggering does nothing to hide his discomfort.

"Is it horrible?" he winces. "Does it look like me? As long as it doesn't look like me, then there's no voodoo risk."

He leans forward and presses his lips to the ornate little pipe protruding from the pot that sits before him on a saucer: it's maté, a rejuvenating South American tea that resembles wholegrain mustard from where I'm sitting. Then he lights another cigarette.

His appearance suggests a bohemian draft dodger. He is wearing jeans, and a beaten-up old khaki jacket over a black T-shirt; on one wrist hangs a blue plastic bangle. His armchair is pointed very deliberately toward the window, and he is drenched in the stark morning light. He is a tranquil presence, give or take the odd fit of throaty, convulsive laughter.

Mortensen, who is 43 years old, was a latecomer to the Lord of the Rings cast. After the young Irish actor Stuart Townsend was shown the door two weeks into shooting, the search was on for a replacement of comparable intensity to assume the role of the warrior-king Aragorn. Mortensen received a call asking him if he fancied committing immediately to the three-film, 18-month production in New Zealand.

"It felt like a challenge," he says now. "Were I not to accept it, I might feel, if only privately, that I had been a coward. I'm always tentative when it comes to the point of committing because without that little bit of fear you're not going to be nudged into doing anything."

Challenges, cowardice, fear. It's a very Aragorn way of looking at things. I had come to our meeting ready to say to Mortensen: all this Lord of the Rings stuff is a bit daft, no? All that nonsense about "Go to the mines of Moria, past Mount Doom, and take a left at the Multi-Storey Car Park of Zumblar".

But once I listen to him explaining in carefully measured sentences the parallels between Aragorn the warrior and Viggo Mortensen the actor, such an enquiry is rendered impertinent. I could even say that, despite hooting several times during the film's po-faced exposition scenes, I am swept along by his explanations.

"I hope you feel to some degree Aragorn's sense of hesitation. On a practical level as an actor, that was already with me when I arrived in New Zealand. I had read enough on the plane to see that the character had misgivings about the burden of the undertaking. He feels the weight of other people's expectations; it's one thing for someone to tell you that you're capable but it's quite another for you to know it yourself. I felt that in Aragorn, and I felt it too as an actor: 'You've hired me 'cos you think I can do it but privately I'm not sure'."

That ambivalence must be interesting to play, I suggest.

"Well, you don't consciously play that, or anything else. 'What are you doing?' 'I'm playing ignorance.'" He unleashes his mighty laugh. "It's just that Aragorn is not quite sure. Not that you play being unsure either." He stops and thinks. "You just ask yourself: what are you sure about? And you find there's a lot missing."

Some of the film's publicity has focused on Mortensen's immersion in his role as some manifestation of actorly eccentricity: "It was rumoured he was living in the forest in Aragorn's torn, mud-stained clothes!" whoops the writer of the press kit, while stories of Mortensen camping out under the stars, refusing to remove his sword from his back during mealtimes, and requesting that a displaced tooth be glued back into his mouth, have begun doing the rounds. But when you meet Mortensen, these anecdotes seem not only plausible but logical. Of course it would make sense to sleep in the forest to play a child of nature, just as he trained alone, rather than with his co-stars, in preparation for his role as Demi Moore's sadistic sergeant in GI Jane, the better to comprehend his character's sense of isolation.

"I thought of the New Zealand landscape as one of my acting partners," he says. "Those forests and mountains - Aragorn knows them. He understands the language of the birds and beasts." He takes a long drag on his cigarette. "He has a special reverence for trees."

You sense that what Mortensen prizes in Aragorn, he prizes in himself. When he tells me that Aragorn understands the value of "stretching yourself, being passionate about other cultures and languages", I discern only the thinnest of veils separating observation from autobiography. Mortensen does, after all, speak three languages, having lived in Venezuela, Argentina and Denmark after a childhood spent in Manhattan with his Danish mother and American father.

One crucial difference is that Aragorn is an autonomous, unfettered individual, whereas Mortensen has always wrestled with the fact that whatever he does as an actor, whatever heights he reaches, he will always, come post-production, be the subject of a director's whims and fancies.

"Like it or not," he wrote back in 1995, "for most of the people involved, their job is completed by others in the windowless rooms of editing bays and sound stages." It's getting easier for him now. "I used to be very bothered. Now I have to consciously say: it's what I did that matters, and walk away."

His work in GI Jane was brave - he brought understatement to the kind of role that offers grandstanding opportunities on a silver platter. But he seemed swamped by the hyperbolic excesses of that superficial film. Conversely, he can appear in a movie for no more than 10 minutes - his whimpering informer in Carlito's Way (1993) or blithely arrogant sportswear magnate in Daylight (1996) - and steal it from beneath the noses of the leads.

"I just try to find something interesting until I run out of money," he says candidly. "And then I have to find the best from whatever I'm lucky enough to get. Daylight was a case in point. But once you do something, no matter what your reasons, it's your responsibility to give your best work. Then every so often there's something where, if someone said 'I'd like to see what you do', I might say, 'Have a look at this'." When I press him for an example, he squirms. "Now I've set myself a trap," he rebukes himself. "I think I'll move on."

He might have trouble receiving praise, or dishing it out to himself. He's proud of having worked on Gus Van Sant's 1998 remake of Psycho, but when I tell him I adored the film, he seems unsure.

"You did?" he says suspiciously.

It becomes clear precisely how much Mortensen values his work when I leave - or rather, after I have left. I'm crossing the hotel lobby when he appears out of nowhere, still barefoot, still nursing his pot of maté on its tiny saucer.

"I was looking for you," he says intently, drawing me to one side. What has caused him to race down from his suite, probably giving several PR assistants heart attacks in the process, is the urge to impress upon me that one director has inspired him more than any other he has worked with - Philip Ridley, the British film-maker who cast Mortensen in his Lynchian adult fairy-tales, The Reflecting Skin (1990) and The Passion of Darkly Noon (1995).

"That man will never sell out," he enthuses, "because his vision is unique."

As well as genuinely praising Ridley, I think Mortensen is also reminding me what he himself aspires to, as if that could have been in doubt. As we say goodbye for the second time, he calls across the lobby at me.

"You will mention Philip, won't you?"

I will, I reply. And I have. Sort of.