Brooding, intense, a leader among men - it's hard to tell where Viggo Mortensen begins and his Lord of the Rings alter ego ends. A case of life imitating art? No, Viggo says, he has always been like that.

I am being seduced by royalty. And not your garden variety Windsor, either. Admittedly, he looks more like a gypsy in his earthy tunic repaired to within an inch of its life, his hands and nails bearing the ingrained grit of a farmer. But he's a king all right: the King, the Lord of Men. He is Aragorn, son of Arathorn, and any minute now he's going to reach out one of those taut brown arms, lift me up on his trusty steed and whisk me away from all this...

All right, I'm dreaming, but if the star of the world's most famous fantasy - you know, J.R.R. Tolkien's Lord of the Rings, book of the century, film trilogy of the millennium - can't launch a thousand fantasies, who can? For even sitting in a plastic chair under the unflattering glare of fluorescent, in a drab office at Miramar Productions' headquarters in New Zealand, Viggo Mortensen is by far the dishiest bloke ever to have donned a crown - right down to the itsy scar above his lip (a combination of a fist and a barbed wire fence. "I was 17 and so drunk that I didn't even need an anaesthetic when the doctor sewed me up").

Fame has come late to the 45-year-old actor, but from the moment he entered Middle Earth in the guise of the mysterious ranger in 2001's Fellowship of the Ring he set hearts pounding. By the time the curtain goes down on part three, The Return of the King, our hero will have shed the traveller's gear, led his charges into yet another heroic battle and taken his rightful place as ruler of men - and Viggo Mortensen will have sealed his fate as a Hollywood heart-throb, no matter how much he'd prefer otherwise.

Up close, this enigmatic Danish-American is slighter and less imposing than his celluloid creation, but he is cut from the same cloth. It helps that he's wearing Aragorn's dishevelled ranger costume, his chiselled features framed by that curiously sexy straggly hair and beard - he is in Wellington to fine-tune scenes for the final film - but it's his manner that shouts "strong, silent type". He mulls over every question, no matter how mundane; his soft American drone is strangely hypnotic, making his rather tortuous replies seem more profound than they really are. Like his alter ego, Mortensen gives the impression he knows a hell of a lot more than he'll ever tell.

In fact, as others have observed, he has so much in common with the inscrutable Aragorn that it is hard to know - elves and wizards aside - where one ends and the other begins. Mortensen is well travelled, speaks several languages, likes solitary pursuits and is wont to ponder the meaning of life. What some might see as LOTR affectation - such as rocking up to interviews barefoot - is actually the real deal. "No, I'm not doing a hobbit thing or a Peter Jackson thing," he told a reporter who queried his lack of footwear. "I'm doing a Viggo Mortensen thing."

But he wasn't director Peter Jackson's first choice for Aragorn, nor even the second. The role initially belonged to Irish actor Stuart Townsend, but though he is certainly no cave troll in the looks department, two weeks into shooting Jackson realised his mistake - Townsend was simply too young for the man who would be king. The word is a call was then put in to Daniel Day Lewis, but he wasn't interested.

Enter Mortensen. It was the offer of a lifetime, but the notoriously indecisive actor - "I could easily use 18 months to think things over" - almost said no, too. Apart from the short notice, the epic $450 million trilogy was being shot back-to-back, requiring an initial commitment of 16 months in NZ and taking him away from his 11-year-old son, Henry. And although he had heard of Tolkien, he hadn't read the books, let alone the scripts. He asked for a couple of hours to think about it.

Fortunately Henry, who was at home with his dad in Venice, California, was Tolkien-savvy. "He said Strider (Aragorn) was the coolest character in the book and told me I had to do it," says Mortensen.

But when he next spoke to Jackson the director wasn't sure he had his man. "We needed him on the Tuesday and it was the previous Thursday," recalls Jackson. "He asked this question I couldn't answer, a difficult question about Aragorn's psychology. There were long silences and I thought 'This isn't going well', and was about to hang up to find another actor when he said, 'I guess I'll see you on Tuesday'.

"Occasionally, fate does step in. In hindsight he was the one person in this world who was perfect for this film. He is Aragorn. And we have his son Henry to thank."

But the decision was about more than pleasing his son. "I guess in the end I did it because I would feel I had been chicken shit really," says Mortensen. "I thought I would probably regret it later (and) not because it's a big Hollywood movie because those things are extremely uninteresting to me."

So he jumped on a plane the next day and arrived at the Wellington set barefoot, wondering what the hell he had let himself in for. The filmmakers, however, were immediately impressed. "We knew we were blessed when he arrived carrying a copy of the Volsunga Saga (one of the Nordic tales that inspired Tolkien's work)," says co-scriptwriter Philippa Boyens. "Viggo not only has a great actor's sense of bringing his character to life, but also an innate understanding of the warrior code and Tolkien's philosophy of heroism."

Yeah, so he was in touch with his inner Viking, but even Jackson teases that Boyens and Fran Walsh, his wife and collaborator, had less noble motives: "I think they were attracted to his tight pants in GI Jane." It's a joke of course, given the director's regard for Mortensen's ability, but there's a grain of truth in it. Jackson had a stellar ensemble, from English sirs Ian McKellen (Gandalf) and Ian Holm (Bilbo) to the luminescent blue-eyed Elijah Wood in the pivotal role of Frodo, but he was short a hunk.

Pre Lord of the Rings there was little on Mortensen's CV to suggest blockbuster material. Sure, he had courted Nicole Kidman (The Portrait of a Lady); snogged Gwyneth Paltrow (A Perfect Murder) and Diane Lane (A Walk on the Moon); spent time in rehab with Sandra Bullock (28 Days); traded blows with Sean Penn (The Indian Runner) and terrorised Demi Moore (GI Jane). He had even won critical praise in the most dire of these films. But his career had moved at a steady snail's pace: his first three parts, including a gig in Woody Allen's The Purple Rose of Cairo, ended up on the cutting-room floor, inspiring him to write a poem that includes the lines:

The man you were
For one short season
Has been pruned
Removed
To a well-groomed graveyard
That smells like popcorn

He finally made it on to the screen in 1985 as Alexander Godunov's Amish brother in Witness. And now, of course, he is a star. Still, Mortensen is disarmingly nonplussed about his impact in the trilogy, "I haven't done things differently in the second or the third films as a result of the success of The Fellowship of the Ring. Nor does it matter to me that the third film is called The Return of the King." He doesn't want to seem churlish - he knows the mythical warrior has opened doors for him - but for him it's all about the process. He is fond of the Robert Louis Stevenson quote. "To travel hopefully is a better thing than to arrive and the true success is to labour." In other words, so long as he makes the most of the journey the outcome isn't as important, no matter how many plaudits, or female admirers.

Maybe that's because despite his notoriety for immersing himself in a role - from insisting on training apart from the cast to capture the isolation of the sadistic sergeant in GI Jane, to sleeping with Aragorn's sword throughout the Rings shoot - Mortensen doesn't have to be in a movie to be creative. As Elijah Wood observed, the guy is a regular Renaissance Man. He is a published poet, an accomplished jazz musician, and had several exhibitions of photographs and paintings (that's his art in A Perfect Murder) under his belt before his star ascended - he just gets more women to the openings these days. "I know people are curious to see if an actor can do anything else and I don't blame them. But I can't not do something because I'm worried about what people will think."

The first of three boys born in New York to an American mother and Danish father, Mortensen and family trouped around the world - Venezuela, Argentina, Denmark then back to the US - as his restless father moved from job to job. As a result he is fluent in Spanish and Danish and he learned to adapt to any situation. "I'm good at making myself at home wherever I am. I don't know if I'm as good in a social gathering. I could spend days by myself." He pursued the arts from an early age, too. "My mother says I'd always go around with a pencil and draw. I think everybody naturally makes things, unless they have it beaten out of them by the school system."

He studied politics at university in New York (perhaps explaining his tendency to talk about Tolkien's opus in terms of the importance of inter-racial harmony), before hitting the road again. He sold roses on the streets in Denmark for a while, then returned to the US to take up acting. Bartending and driving trucks helped pay the rent in between parts.

He met Henry's mother, Exene Cervenka, actor and singer with LA punk band X, on the set of the 1987 evangelical-scandal flick Salvation. They divorced years ago but remain friends. They have collaborated on multi-media projects and he waxes lyrical about her abilities. He has been linked with other women, but doesn't appear to have a significant other. Where would he find time amid all that creativity? "It's a tough one to put someone through all of that," he says, laughing. Henry - who ended up with minor parts in LOTR as a young warrior and an orc - is clearly the most important person in his life. "I don't know what I'd do without him."

Mortensen comes across as far too deep for the Hollywood treadmill, perfectly cast as a dreamy poet or avant-garde artist, but hardly the stuff of headline-grabbing celebrity soundbites. And the stories of how he went about getting under the skin of Aragorn don't make him sound like a scintillating dinner guest, either. He camped in the woods on his own whenever schedules permitted, insisted on carrying his sword at all times and even mended his own costume. "It was an unusual request," says costume designer Ngila Dickson. "But it was important to Viggo and, therefore, important to us. Who knows, perhaps it was because he washed and repaired Aragorn's clothes himself that he so perfectly came to inhabit them."

Mortensen says it's no secret he likes to be well prepared - he just had more time on such a long shoot - but believes such stories have been exaggerated. "On the weekends sometimes I'd go camping and it was probably helpful to remind me of what Aragorn was familiar with, but I've always been like that, since I was a boy. When I'm by myself in some natural setting I never feel there's a second wasted." And though he did look after his sword and costume, "I took off the clothes when I slept and I did shower!"

His sense of humour is slow to emerge, but it's there. Asked a few years back about his apparently famous "good walk", he laughed at such a ludicrous notion: "What's a good walk? I stay in a straight line I think. That's a good thing to aim for." It's precisely because we've cast him as the archetypal strong silent hero that when he does pop up as he did unexpectedly on the MTV awards playing the lad, threatening to run a sword through Peter Jackson, it's somehow funnier than a Robin Williams monologue.

And behind the scenes it didn't take long for him to inspire the kind of loyalty no existential bore could hope for. When it looked like Jackson had missed his chance for a crucial shot in the second film, for instance, Mortensen persuaded the cast to camp overnight on the mountainside to catch the next day's rays. "We flew in steaks and made a night of it," says producer Barrie Osborne. "It was this kind of dedication and leadership that inspired our younger actors."

And he was just as at home with the horse handler and swordmaster (he was, predictably, great at riding and fighting, losing a tooth and breaking toes in the process) as he was with the actors. Each presumably connected at a different level with this multi-layered personality. As Bernard Hill (King Theoden) puts it: "He's like a multiplex. He's a poet, a singer, a painter...not that we're jealous! He's a great guy."

He's also probably the most unlikely star ever to have been made into a fast-food action figure. Although Mortensen now sees LOTR as an unexpected gift, the fame it has afforded him can be "stressful and a little weird."

"Maybe I should go hide myself for a couple of years in Middle Earth."

Can I come too?