NO, scratch all that. Ewan McGregor - when he's playing Iago in the Donmar Warehouse's stage version of Othello - is a malicious, deceitful swine. The rest of the time, he is the same funny, easygoing, generous guy he has always been. Today he is being open and honest in a restaurant in St John's Wood in London - a boundary ball away from Lord's Cricket Ground, within hearing distance of Abbey Road and literally around the corner from his family home.

McGregor doesn't make a big show of an entrance. There is no agent, publicist or stylist in tow. Just the 36-year-old actor, dressed down in an old black pullover, black jeans and a battered leather jacket. A batch of good reviews for his stage performance lends him a certain Shakespearean distinction, but it will be back to business as usual in a couple of weeks' time when the run of Othello comes to an end. Besides, any sense of gravitas bestowed by his theatrical credentials disappears as soon as we start talking about Cassandra's Dream, the film he made with Woody Allen and which opens the Glasgow Film Festival this week.

"Every young boy wants to get a good Woody," he cackles, never one to knowingly miss out on a cheap innuendo. The film casts him as Ian, a cocksure young cockney who wants to break free from his father's small restaurant by investing in a Californian hotel empire. Meanwhile, his brother, played by Colin Farrell, is in serious debt after a losing streak at the poker table. The money comes, but at a price: cue guilt, despair and the death of brotherly love. Annie Hall this is not. It's as far from Allen's "early funny ones" as East London lock-ups are from the Manhattan skyline.

McGregor, however, couldn't care less where the movie is set. "Having worked with Allen once, I'd work with him again in the blink of an eye. As an actor, you really have to raise your game. I wanted to work with him because there are only a few grandmasters of film-making, and he's one of them. Tim Burton is another person who completely understands how he makes films. Ridley Scott too. They know what they're doing, and it's nice to be along for the ride."

Working with Allen, McGregor found, meant quickly sorting out myths from reality. He was offered Cassandra's Dream after a rather brusque meeting in the director's New York office. "I went to the edit suite where he has cut every movie he's made since the 1970s," he remembers. "All his jazz records are along one side of the screening room and it's full of brown carpets. The place just feels like him, like Woody Allen, as it should be. All he said was, I'm making a film in London. I've seen some of your work. There's a part I think you might be good for. We're making the film this summer. I just wanted to see you in person.' And that was it. It was very clear that he was done and I should leave. So I did."

Allen shoots without rehearsals and with a minimal number of takes. To keep on the ball, each morning during make-up, McGregor and Farrell would run through the day's dialogue-heavy scenes together. Perhaps it's no surprise that the pair got on so well. Both are Celtic sons who gained a reputation for living it up and over-indulging on the trappings of fame once they hit it big (although McGregor, who reckons he worked his way up to being a borderline alcoholic, hasn't touched a drop for seven years).

"I adored Colin from the moment I met him," McGregor says. "I'd read a lot about him over the years and, because I used to be a big drinker, I recognised some of the things he'd said in myself. Because we were both in LA, I got in touch and asked him to our house there one Sunday. We were having lunch with the kids when he arrived in this great big Bronco truck, but he got out in his pyjamas. He was so lovely with my girls, he's such a sweet soul. Colin is a very quiet character now. He focuses on the work and has incredible commitment. He'd never falter on his lines while I was fluffing away."

As he talks, McGregor cuts into a fairly rare steak, leaving the side salad for later. I wonder if I should point out that almost every time he dunks his fries then pops them into his mouth, he drips hollandaise sauce on to his pullover. Instead, I let him get into full flow about that kid-in-a-sweetie-shop feeling that comes with the first rush of success. One minute he was a boy from Crieff studying drama; the next, as Renton in Trainspotting, he was the poster boy of the Britpop generation. International stardom came as he serenaded Nicole Kidman in Moulin Rouge and waved Obi-Wan Kenobi's light sabre in Star Wars.

"When Trainspotting came out, it was the 1990s, Oasis, Blur, Kate Moss, way-hay, f***ing unbelievable! But there were casualties there, people who didn't make it through. I'm lucky now to understand how to defend myself from it if I need to. The most basic thing is the work. If you can make that the centre of it all, then you're in good shape. As soon as any of the other trappings of it become more important to you, you're f***ed. The most obvious example of it in the world at the moment is with Britney Spears. She's not without responsibility for her actions - of course not - but the pressures of that amount of fame on her shoulders when she was 15 or 16? She was a child. And the whole part of being a child is that you shouldn't have to deal with those things yet. I feel sad for her because I look at those pictures of ambulances outside her house and there are hundreds of paparazzi photographers. It makes me sick."

So what is the work that's keeping him from a similar fate? Already in the can is Incendiary, in which he plays the adulterous lover of Michelle Williams, whose husband and son are killed in a suicide bombing at a football match. He shares the screen again with Williams in The List, this time playing an accountant who is seduced into a sex club then set up for a heist by Hugh Jackman. He has also done a voiceover role for stop-motion animation spoof Jackboots On Whitehall, in which the Scots save the English after a second world war German invasion. In April, he begins work on dark comedy I Love You Phillip Morris, co-starring as the prison lover of Jim Carrey. For this one, he's as excited about the script - written by Bad Santa duo Glenn Ficara and John Requa - as he is about shooting in New Orleans for the first time.

And then there is Othello. Night after night of transforming into Shakespeare's most morally complex, hard-to-fathom villain. Stepping into buckled shoes previously filled by Kenneth Branagh, Ian McKellen and Henry Irving. Mind you, that's probably nothing to the man whose last West End stage outing involved taking the Marlon Brando role in Guys And Dolls. But surely the theatre critics were sharpening their knives as yet another "Hollywood" star took a stage sabbatical?

"I believe in myself as an actor and I belong as much in the theatre as I do on the film screen," McGregor counters. "Although I don't do as much of it as I do filmmaking, it's still my job. This is the hardest thing I have ever done. To begin with, I found the process of it absolutely terrifying but it paid off. I don't concern myself with what people will say or that kind of pressure. The pressure is in playing the part itself."

Of course, acting isn't all that McGregor does now. Last year, he and best friend Charley Boorman set out on their second major motorbike tour, heading from John O'Groats to Cape Town. The route took them through the heart of Africa, which was fascinating in itself but not quite as satisfying for viewers of the subsequent television series as their earlier east-to-west Long Way Round trip.

"The rush of it upset me a bit," he admits. "I wanted to get to Cape Town on that final day because I had a holiday booked with my family, and after three months of not seeing them, I wanted to fly up to Kenya and spend two weeks sharing some of the Africa I'd learned about."

But his wife, Eve, came along for part of the trip - an unnecessary risk, surely, with three young daughters back home?

"I was worried when we were riding around in London behind her when we first started, because I suddenly realised how vulnerable she was," he concedes. "But not in Africa. I never understood why there was a problem about Eve coming - I still don't."

Not even when it seemed to rock the two-men-together dynamic he had with Boorman?

"But Charley has known Eve for as long as he has known me. He has always known me with her. She's my wife, so I would put her before anyone else. So it's difficult for me to see the problem. We rode for a week, doing 1500 miles through Zambia and Malawi. She loved the freedom and adventure of it, stopping and meeting people."

There is talk of completing the motorbike trilogy with what could become Long Way Up, heading north through South America. But if that doesn't pan out, at least McGregor got the chance to visit Brazil for a shoot for men's fragrance Adventure by Davidoff. Celebrity endorsement of a smell?

The knife and fork go down, and lunch pauses for a moment. "The smell of adventure " McGregor is not easily offended by the accusation. "Yeah, we tried to make it smell like my boots from the bike trips. Actually, it's quite ehm I'm not really good at describing it peaty and earthy." But there is a slightly more serious and definitely more interesting side to his decision to become the face of a male perfume. And it says something about how he regards - and exploits - his public persona.

"It's not something I would have done in the past, but they said that the concept was completely based on Long Way Round and that experience of adventure. If Charley and I have inspired people to travel and see the world, then I'm very proud of that. In Africa, who's on the Oscars list bears no relevance to their lives because they've got no medicine or no food. It's good to remember that. Those TV shows are me - they're not some script and I'm not playing a part. So the Adventure fragrance does more directly relate to me as opposed to the image of an actor."

For McGregor, charity work seems to be more than your typical celebrity conscience-cleanser. A long-time supporter of Unicef and the Children's Hospice Association Scotland (CHAS), in March he and Texas singer Sharleen Spiteri will be reviving their now annual (but slightly postponed) Burns Supper in London, raising money for children's charities. ("I can do every line of Iago but I can't remember the Address To The Haggis," he admits.) In the footage from his two motorbike tours, it was the plight of children that most openly affected him. Maybe that comes of being the relatively young father of three girls - one just turned 12, one six and one adopted from Mongolia two years ago. Given his Britney Spears comments, it's not surprising to learn that this area of his life is strictly off the interview radar.

Nevertheless, McGregor is no soft touch when it comes to promotional events. There's often a personal, politicised-with-a-small-p agenda involved on his part. Like when he agreed to go to Iraq last October to present Staff Sergeant Michelle Cunningham, a bomb disposal expert, with a televised Pride Of Britain Award.

McGregor's older brother, Colin, served with an RAF Tornado squadron in the Gulf - which is probably why, although critical of the West's involvement in Iraq, the actor remains sympathetic towards the soldiers he met there.

"The place we flew to in Kuwait before going up into Basra was where my brother was based," he explains. "Since I made Black Hawk Down, I have had this fascination about soldiers and warfare, how people deal with it and how the human condition survives through it - if it does or it doesn't. I met the guys from the rifle division that had been holding Basra Palace, and you could see in their eyes that they'd been through it. They'd been getting mortared 16 times a day, really in the thick of it.

"It was an experience to land in a Hercules at Basra International Airport. Because of the possibility of someone firing an RPG rocket-propelled grenade at the plane, you come in very high and you can't do a normal landing. The plane goes into blackout, and you have to put on your helmet and your body armour. It does this dive with sharp turns to lose height, and then they just bump it down. In pitch black. My mum despairs. If I'm not riding a bike through Sudan "

The meal is over, the plates are cleared, and we're on to the coffee. It's time for McGregor to indulge in a more reflective mood.

"I'm happy, I really am," he says. "I'm more excited about my work than I've ever been. I mean, I could be going for another lead and thinking, shit, I'm not getting the parts that so-and-so is getting. But if you're chasing that sort of thing, I don't know that you'll ever be happy. This feels like another fresh start because I've had the trip, I've had the play, and I haven't been on a film set since this time last year. I really feel like it's time to get back to work. I'm inspired by having seen There Will Be Blood, seeing Daniel Day-Lewis's performance and realising that there are depths to this job that I do that I've yet to plumb."

Ah, but Daniel Day-Lewis isn't on a stamp. In America, Ewan McGregor is now part of a Star Wars postage set.

"But the thing is," he argues, "it's Obi-Wan Kenobi who is on the stamp, not me."

Nevertheless, people are now licking his backside.

"They've been doing that for years, love," he laughs. "They've been doing that for years."

Cassandra's Dream opens the Glasgow Film Festival on Thursday and goes on general release on May 9