From the speed with which all the tickets for Othello at the Donmar Warehouse in London have gone, it would be quite possible to argue that Ewan McGregor is bigger box office than Nicole Kidman or Gwyneth Paltrow. Both actresses have starred at the small but influential venue in the past decade; both packed it out and received glowing reviews (Kidman’s performance being famously described as “pure theatrical Viagra”). But neither show shifted seats as fast as this one, which sold out in less than six hours.

McGregor, who was made famous by the film of Trainspotting in 1996, is playing Iago, the embodiment of jealousy, and describes this as quite simply the hardest thing he has ever done. No ifs or buts here, no maybes or on-the-other-hands. Ask him how it compares with life’s other challenges, like fatherhood, or going down to the foot of Africa on a motorbike, or skimming the hills of Scotland in his brother’s Tornado jet, and none gets a look-in. Nor do the big movie roles – such as Christian in Moulin Rouge! or Obi-Wan Kenobi in the Star Wars franchise – which have maintained his profile these past 11 years.

In playing Iago, the biggest non-title role in Shakespeare, the 36-year-old McGregor becomes the latest of many actors known mainly for film or TV work to take on a serious West End stage production. Apart from Kidman and Paltrow, there have been appearances by such major players as Madonna, Kevin Spacey, Christian Slater, Woody Harrelson, Patrick Stewart and Stockard Channing. In some cases, such as Stewart’s, there was already a body of stage work before the arrival of Jean-Luc Picard and Star Trek: the Next Generation; in others, like Harrelson’s, it was TV and films almost from the start. McGregor falls into the second category, his career having been influenced by the success he scored even before Trainspotting. He was still at the Guildhall School of Music and Drama, six months from graduation, when he landed a lead role in Dennis Potter’s six-part TV classic, Lipstick on Your Collar.

The startling result – startling when you consider that he’s about to tackle one of the greatest Shakespearian monsters – is that he’s done very little stage acting as a professional. There was the musical Guys and Dolls two years ago, and his title role in a revival of David Halliwell’s cult classic Little Malcolm and his Struggle Against the Eunuchs, but these were six years apart, punctuating a film career that has become busy rather than dazzling. Last year alone there were three: Scenes of a Sexual Nature, Miss Potter and Stormbreaker. So either he’s asking for trouble, or he’s riding his luck as gladly as he rides his BMW R1150GS Adventure, or else he is buoyed up by the faith being placed in him by the Donmar’s director, Michael Grandage. Perhaps it’s a little of all three. Either way, if a man can be said to bound warily into a room to discuss his situation, then that is what he does.

“I’ve never had to work harder than this,” he says. “I mean, the sheer size of it. Since being in South Africa [for the most recent TV motorbike epic] in August, I’ve done almost nothing except work on the play and prepare myself for the part. Everyone was saying, ‘Oh you’re in London, can we have you for this, or this?’, which was nice, but I just had to say, ‘Sorry but no,’ because all I wanted to do was sit at home and learn the script.”

I ask him if he is trying to prove something, and he breaks into a big, open smile and runs a hand back through his tufty hair. It seems a fair question. Spacey, Stewart and others constantly talk about the different challenges and rewards of live work, and make it sound as if an actor’s professional virility remains unproven if he spends a lifetime shirking these summits in favour of lower but more lucrative pastures. “I’m just approaching it as another acting job,” McGregor replies, “and that’s how I hope people will come to see it. I’m an actor, and that’s how I’ve approached all the work I’ve ever done. So, no, I’m not trying to prove anything, except to be fully involved in the production, and to nail Iago.”

No one is doubting that he’s up for it. He’s often in before anyone else, full of observations on text and character during rehearsals. Grandage says he’s been trying to lure McGregor back to the stage since he directed him as the hunky gambler Sky Masterson in Guys and Dolls at the Piccadilly Theatre. “Ewan,” says the director, “has a head start [for Iago] in that he looks like a guy who is good at male company. Good at it, and used to it.” McGregor looks lean and alert. He may be a more mellow version of laddish now, but he is recognisably the same peddler of slightly risky charm that he was as Nick Leeson, Barings Bank’s nemesis in Rogue Trader, eight years ago.

There’s no doubting the truth of what Grandage says. For four and a half million viewers of his TV journeys Long Way Down and the earlier Long Way Round, McGregor has become the very image of the biker-buddy, bonded but restless, circling the globe with his friend Charley Boorman in a because-it’s-there sort of way. But for some people, critics and civilians alike, the whole thing looks suspect. Boorman comes across as a bit of a prat, showing off to the camera and making the most of his famous friend. This is unexpected, when you consider that he is the son of John Boorman, who made one of the finest feature films ever about men up against it in the wild, Deliverance, in 1972; Charley, then six, even appeared in it. But these two don’t seem to be searching for anything more than ten hours’ telly. McGregor comes out better from the inevitable comparison – the Peter Fonda rather than the Dennis Hopper, the James Bolam rather than the Rodney Bewes.

There is other baggage that McGregor carries – the kind you bring back from extended galactic touring. “I’m not all that aware of it,” he insists. “That’s not to say it’s not there, but I don’t come face to face with it that often. Unless I’m at a premiere, when people want me to sign an Obi-Wan Kenobi poster. Whether people will come to see me because of that, I don’t know. It’s quite weird, because

I honestly have no idea what people’s perception of me as an actor is, whether they accept that it changes from person to person. What I do know is that working to the depth that we are at the moment is terrifying; there’s no point in pretending otherwise. But that shouldn’t be seen as a negative thing – I’ve never been happier.”

He was once rude about Sean Connery for telling Scots how they should feel about their country while not living there. McGregor regards himself as an adopted Londoner, a passionate one who considers it the best city going, and has now lived in it for half his life. You could say he’s done his bit for the Auld Alliance by marrying a French woman in 1995 – Eve Mavrakis, a production designer whom he met during an appearance on the TV series Kavanagh QC. They have three daughters, one of whom, Jamiyan, was adopted two years ago from Mongolia, when she was four. McGregor tends not to talk about his family, particularly the adoption. “I simply prefer to keep it private. Once I start to discuss it, it opens it out and becomes, well, not private any more.”

A look of mild contrition passes briskly across his face when he thinks of Connery and the question of Scottish patriotism. “I’ve got a very big mouth at times,” he goes on, “and I’m sure I said something stupid about him. But it’s not really my place. The fact is that I don’t really know what’s right or wrong for Scotland. I think it’s up to the people who live there. It seems to me that the Scottish Parliament has been reasonably successful. As I say, it’s up to them, but, you know, if they did choose to make Scotland an independent country, I’d still live here. I like the idea of Great Britain, of us all being together. If that doesn’t work for people financially, politically, if there is a feeling that Scottish wealth is being bled into England, then… I don’t know, I’m so ignorant about it.”

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