Focus Features

Most actors will agree on the value of appearing enigmatic. But there is enigmatic and then there is Viggo Mortensen, currently appearing in David Cronenberg's Eastern Promises (his second film for the director after 2004's A History of Violence). The apparent severity of this serious and impassioned actor extends to his Nordic features: he has hard blue eyes and a pair of cheekbones that could slice bread. And now, one birthday shy of 50, he has achieved what in movie-career terms could be considered the impossible. He appeared as Aragorn in the Lord of the Rings trilogy (2001-03), which — it is currently estimated — everyone in the world has seen; and yet, more than 15 years after his devastating 'breakthrough' performance as a shell-shocked Vietnam veteran in Sean Penn's directorial debut The Indian Runner (1990), Mortensen remains possibly the biggest unknown actor in Hollywood. He's the star who isn't there.

Even when Mortensen consented to a blockbuster, it's significant that this was one filmed far from LA, in the New Zealand wilderness. He is something of a roamer, having lived in Venezuela, Argentina and Denmark after a childhood spent in Manhattan with his Danish mother and American father, and in Aragorn Mortensen seemed to find some kind of kindred spirit. "I thought of the New Zealand landscape as one of my acting partners," he said. "Those forests and mountains-Aragorn knows them. He understands the language of the birds and beasts. He has a special reverence for trees."

Perhaps the fact that Mortensen was only one element within the sprawling tapestry of The Lord of the Rings has helped him remain incognito. Elusiveness combined with intensity has always been his stock-in-trade. After an eye-catching early appearance in Witness (1985), he spent a few years grafting in films that didn't reach wider audiences. Was that why it was such a revelation when he finally left the starting blocks as an actor in The Indian Runner? As Frank, the returning soldier who can't keep to the straight and narrow, Mortensen was frighteningly unknowable.

When you feel his anger brewing-and it comes out of nowhere, like a freak storm-you want to duck for cover. Sometimes the alarm dissipates; in my favorite scene from the film, he startles a neighbor who has called at his door, yanking her Elvis t-shirt over her face while an inquisitive old coot looks on. Mortensen oscillates between drowsy menace and raucous mania, making you unsure of the scene's intended tone, and of him; it recalls Jack Nicholson's infamous diner scene in Five Easy Pieces, only without the comforting hint of showmanship. And if there is in recent cinema a more convincing scene of psychological torture than the moment when Mortensen rages against a teeny-weeny Patricia Arquette, spattering her with mouthfuls of food, I'd really rather not see it, thank you.

The great role that would build upon Mortensen's work in The Indian Runner didn't come until over a decade later, with A History of Violence-another film that called on him to suggest unchecked reserves of aggression, albeit in a lighter register. Along the way he acted in two adult fairy-tales for the British writer-director Philip Ridley (The Reflecting Skin [1990] and The Passion of Darkly Noon [1995]), work of which he is especially proud. And he contributed some choice character studies to various other films. "I just try to find something interesting until I run out of money," Mortensen once told me. "And then I have to find the best from whatever I'm lucky enough to get. 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 pressed him for an example, he squirmed visibly. "Now I've set myself a trap," he laughed, before changing the subject.

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Most actors will agree on the value of appearing enigmatic. But there is enigmatic and then there is Viggo Mortensen, currently appearing in David Cronenberg's Eastern Promises (his second film for the director after 2004's A History of Violence). The apparent severity of this serious and impassioned actor extends to his Nordic features: he has hard blue eyes and a pair of cheekbones that could slice bread. And now, one birthday shy of 50, he has achieved what in movie-career terms could be considered the impossible. He appeared as Aragorn in the Lord of the Rings trilogy (2001-03), which — it is currently estimated — everyone in the world has seen; and yet, more than 15 years after his devastating 'breakthrough' performance as a shell-shocked Vietnam veteran in Sean Penn's directorial debut The Indian Runner (1990), Mortensen remains possibly the biggest unknown actor in Hollywood. He's the star who isn't there.

Even when Mortensen consented to a blockbuster, it's significant that this was one filmed far from LA, in the New Zealand wilderness. He is something of a roamer, having lived in Venezuela, Argentina and Denmark after a childhood spent in Manhattan with his Danish mother and American father, and in Aragorn Mortensen seemed to find some kind of kindred spirit. "I thought of the New Zealand landscape as one of my acting partners," he said. "Those forests and mountains-Aragorn knows them. He understands the language of the birds and beasts. He has a special reverence for trees."

Perhaps the fact that Mortensen was only one element within the sprawling tapestry of The Lord of the Rings has helped him remain incognito. Elusiveness combined with intensity has always been his stock-in-trade. After an eye-catching early appearance in Witness (1985), he spent a few years grafting in films that didn't reach wider audiences. Was that why it was such a revelation when he finally left the starting blocks as an actor in The Indian Runner? As Frank, the returning soldier who can't keep to the straight and narrow, Mortensen was frighteningly unknowable.

When you feel his anger brewing-and it comes out of nowhere, like a freak storm-you want to duck for cover. Sometimes the alarm dissipates; in my favorite scene from the film, he startles a neighbor who has called at his door, yanking her Elvis t-shirt over her face while an inquisitive old coot looks on. Mortensen oscillates between drowsy menace and raucous mania, making you unsure of the scene's intended tone, and of him; it recalls Jack Nicholson's infamous diner scene in Five Easy Pieces, only without the comforting hint of showmanship. And if there is in recent cinema a more convincing scene of psychological torture than the moment when Mortensen rages against a teeny-weeny Patricia Arquette, spattering her with mouthfuls of food, I'd really rather not see it, thank you.

The great role that would build upon Mortensen's work in The Indian Runner didn't come until over a decade later, with A History of Violence-another film that called on him to suggest unchecked reserves of aggression, albeit in a lighter register. Along the way he acted in two adult fairy-tales for the British writer-director Philip Ridley (The Reflecting Skin [1990] and The Passion of Darkly Noon [1995]), work of which he is especially proud. And he contributed some choice character studies to various other films. "I just try to find something interesting until I run out of money," Mortensen once told me. "And then I have to find the best from whatever I'm lucky enough to get. 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 pressed him for an example, he squirmed visibly. "Now I've set myself a trap," he laughed, before changing the subject.

During the 1990s Mortensen became one of those names that prompted a sigh of relief whenever it appeared on a cast-list. He did brave work in GI Jane (1995), essentially reprising the Louis Gossett Jr. role from An Officer and a Gentleman. And he could appear in a movie for no more than five or ten minutes and steal it from beneath the noses of the leads: his whimpering informer in Carlito's Way (1993), which remains among his best work despite amounting to just one scene, or the blithely arrogant sportswear magnate in the disaster movie Daylight (1996).

But what is it precisely that Viggo Mortensen does? I would venture that he actually does very little on the surface. He prizes research, and immersion. "It was rumoured he was living in the forest in Aragorn's torn, mud-stained clothes!" whooped the press kit for the first Lord of the Rings movie, while stories did the rounds 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.

From some angles, his low-key technique can perplex. During the early stages of A History of Violence, as Mortensen is evoking the happy nothingness of an uneventful life, you might wonder if the actor is doing anything at all. Actually, he's hanging back. He is an ordinary man, and vengeful gangsters have turned up in his home town, claiming to recognize him from the bad old days. He claims they are mistaken. And it is to Mortensen's credit that, despite the movie's giveaway title, we still can't be certain until he reaches breaking point. The scene which confirms the truth is a masterclass in understatement-it's a shot rather than a scene, the merest flicker on Mortensen's face, but you couldn't say it wasn't dynamite. The actor nailed it on his first stab; Cronenberg knew instantly that there was no need for take two.

If anything, their work together on the new Eastern Promises takes this minimalism to its extreme. Mortensen's performance as the conflicted Nikolai, a rising star in London's Russian mafia, seems designed to prove that a wealth of emotion can be wrung from the tiniest gestures. There are no big, demonstrative Oscar moments here, but the actor's presence is riveting; even hidden behind shades, he gives good, charismatic close-up. And remarkably for a 'star', Mortensen is unafraid to be vulnerable and exposed, whether physically (in the film's shocking bath-house knife-fight, in which he is completely naked) or emotionally (the moving final close-up).

Eastern Promises features Mortensen's most dexterous performance to date-menacing and tender, steely and wry-but then each role seems like a giant leap. Next year, he will be seen in a film of CP Taylor's play Good, as an academic whose writings are co-opted by the Nazis as justification for the gas chambers. No matter how outstanding his work, or how successful his films, it's impossible to imagine Mortensen without that customary reticence that makes him such a fine actor and such a reluctant star. Long may he stay off the radar.