They gave Ewan McGregor a motorbike when he finished the new Star Wars film-a gleaming Ducati 748- a grateful gift from producer and director. Then they panicked that he might hurt himself on it and thwart the course of the most lucrative film project ever attempted. Such are the problems of start treatment; an indispensable actor indulged with his every desire, but God forbid anything that threatens the three-movie deal. How they must have sweated when, a few weeks ago, he wrote off a hired bike, a near-catastrophe for actor and project. McGregor laughs as he recalls their disquiet when he roared about the disused Rolls-Royce factory where George Lucas shot his new movie. When they approached him to tell him about the present, he though he was about to get a telling-off for borrowing from petty cash.

He likes to talk his status down if possible, And, true to reputation today there is no sign of a flattery-fattened ego. Scruffily bearded, McGregor is supping Guinness in a trendy bar, looking tired, at times bored, calling Princess Leia Princess Di by mistake, saying that he would love not to have to do these interviews, repeating his spiel about the thrill of making Star Wars Episode I: The Phanotm Menace. McGregor is the pale, soft-faced boy of 28 who stars in more films than is probably good for him, his on-line fanzine for overheated schoolgirls is called The Ewan McGregor Altar. He doesn't read his ecstatic notices lest he become "carried away", but he has been interviewed by every journalist you ever met and has plumbed his soul on every subject: his love for his French wife, Eve, and their three-year-old daughter, Clara; his hedonistic benders; his proud Scottishness and kilted flings; the regular appearance of his penis on screen (Velvet Goldmine and The Pillow Book to name but two) and occasionally at public gatherings. What style. What humbling normality. What a lad! So why, on meeting him, do I not quite get it? Too old perhaps, to be touched by the gentle features that made David Cassidy my generation's poster boy.

Certainly when his mother Carol enters the room, and he leaps up mid-sentence to shout "mum!" as if he has not seen her for years, and she, a pretty, soft-spoken woman, beams and wraps her arms around him, then the puppyish lovability that permeates his work smacks you in the face. McGregor's role as Obi-Wan Kenobi in the new Star Wars prequel is an uncharacteristic one for the young actor. A promise of intergalactic celebrity maybe, but so utterly divorced from what he normally pursues-character, depth, fashionable edginess- that you know a tiny bit of him will be wondering, as his face is printed on a million junior doona covers, whether he has done the right thing. And in one sense we are in luck to catch him on a tired day: what you lose in sparkle you gain in candor. The obligatory gush about the genius of George Lucas, and having to pinch himself once a day on set, which fills the press releases, soon gives way to a more honest strain of reflection.

"The main thing was to try and stay awake, because it went on and on," McGregor says of filming. "I was leaving home at six in the morning and coming back at nine at night, so I never saw my daughter awake." The challenge of the thing, he adds, was more technical and physical, including 2-1/2 weeks of fighting under tense lights in heavy robes…And the magic of science fantasy? "Och, it's disappointing because you're not in space. Once when George Lucas was showing me a vehicle and I asked him how I was going to play the scene, he just looked at me and said, 'None of it's real, you know, Ewan.'"

The relationship between actor and director seems to have been amiably professioinal rather than close: McGregor no doubt valued Lucas' distaste for Hollywood values; Lucas saw in the actor's innocent gaze a magical antidote to the hype and money talk stalking his film. A film in which the real stars are Lucas' special effects wizards rather than any of the actors. McGregor thought long and hard before accepting the role. He eventually went for advice to his uncle and mentor, actor Denis Lawson, who appeared as fighter pilot Wedge Antilles in the Star Wars trilogy. "He told me not to do it," McGregor recalls. "He said he wanted me to have a career after the age of 30. But in the end I wanted to be in it because of what it meant to me." Meaning? "It is essentially a fairytale-the princess, the wizard, the fight against evil; and I love that. Parents don't bother with traditional fairy-tales anymore, so this is what the younger generation has instead. I love the fact that my daughter will be able to see it in a few years and finally understand what her dad does with his life. There's too much swearing and sex in my other work for kids- either that, or they just wouldn't be interested…I mean, take Emma. I would'nt go and see that, never mind kids. I just hope it won't be a hindrance for Clara- you know, my dad was in Star Wars."

But still the doubt gnaws away. He talks about an actor on set who is not seen in the movie because his head is computer-generated. "Poor man, " I sympathise. "Och, maybe not. Maybe I'll end up wishing I had a computer-generated head. Good for him." Nor is he compelled towards the alleged childlike innocence of such fantasy projects, that Spielberg view of film-making as imaginative play. "Whoever talks about there being a sense of wonder on a film set… well, Christ, I wonder where they work. It's usually a bunch of sweaty men swearing at each other and the producer lying to the director and the cast. It's a horrible place to work- full of liars and badly paid crew sweating their balls off, hating the producers. You can only afford a sense of wonder if you're at the top." McGregor has been famously down on Hollywood and its blockbusting set pieces, particularly action movies less august than his own project.

Part of the designated career path for a young actor, however; is to watch your manners and respect the elders: Hollywood is quick to exclude subversives with an ungrateful attitude. The expectation is of bland, mutual flattery from major players, and avoidance of controversial opinions, to both of which McGregor is oblivious. "I don't' see the point in those action movies-they're the biggest sellars and I've no idea why; but Star Wars isn't part of that. I saw the first one when I was six, my uncle was in it. They're kids' films. I fell I love with Princess Leia. I wanted to be Han Solo. I knew the old guy was the wizard character- at six I absolutely got it. I remember standing in my shorts and red socks outside school waiting to be picked up and taken for this treat and all my mates being really jealous." Twenty-Two years later, mates are still the greatest influence on his life. Part of McGregor's reluctance to act the star is his dislike of feeling set apart form the gang.

He is in acting, he has said, because he needs to feel liked and there is a genuine gregariousness in his preference for ensemble work: Shallow Grave and Trainspotting are really sophisticated buddy movies. In his life his gang is crucial too. Especially the group of friends which includes actor Jude Law and Trainspotting co-star Jonny Lee Miller, who have formed a production company, Natural Nylon, through which McGregor has just directed his first short film. Had his success divided them? "We all made it as the same time, so how could it?" he replies, as if blind to any difference. On the film set, where a strict hierarchy is measured in hangers-on and trailer size, McGregor establishes solidarity with the grips and cameramen. It's not about politics, but about a fear he might one day lose touch with what matters. During breaks in filming he plays backgammon tournaments with his driver. While filming his big Hollywood break, A Life Less Ordinary, he became so depressed he called his uncle and told him he wanted to do a play in London, something interesting, no big deal, he wanted to be scared again rather than just successful. On one small budget British film (which he refuses to name), he walked off-set to protest at the bad treatment of a crew that had been slogging for 10 days without a break. McGregor's background, however, is hardly radical. He grew up in Perthshire, Scotland, the second son of two teachers. It was, he says, "perfectly happy."

Enthralled by his glamorous uncle Denis, star of Bill Forsyth's Local Hero when Ewan was 12, he left school at 16, attended a local drama course, worked at Perth Rep, then moved to London, and still talks of paying back his parents for always believing he would make it. There is an ocean of sentimentality here, but no false modesty; he loves watching himself on screen, thrills at seeing himself living his childhood fantasy, especially-one imagines-since the camera casts a honeyed douceur over an uremarkable-looking Scot. This physical transformation as much as aptitude and workholism, may have speeded his path. But, while he worked in restaurants and on fish farms in his college holidays, he never doubted his destiny. "I was lucky, but I never doubted it would all come to me. I was confident to the point of abstract arrogance." He left the Guildhall School of Music and Drama early to play the Elvis-obsessed clerk in Dennis Potter's 1993 Lipstick on Your Collar: Potter told him to guard his talent fiercely and not take the first fat offer that came along. ("He was wonderful to me, but horrendously rude to waiters in restaurants.")

Then came a TV play, followed by Alex, the cocky journalist in the inky black comedy Shallow Grave- the first of three collaborations with director Danny Boyle. "It was kick, bollock and scramble making that…I had no idea what it was going to look like, but I loved that edge." It was, however, not as demanding as being the centre of scandal following his depiction of the heroin addicted anti-hero of Trainspotting; over and again,McGregor reassured critics he never tried smack to get in character, had never done drugs for fear of hurting his parents, and looked so fresh and appealing as he defended the film that its hipness ultimately triumphed over its dubious celebration of the methadone life. "It was the purest film-making experience I've ever had," he says, "because Danny Boyle is the best director I've ever worked with." Then came Emma, Brassed Off and A Life Less Ordinary, and on to film-star status. For McGregor, Star Wars is all about payoff. "If this film is as big as it looks," he declares, "I can spend the rest of my life doing the work I want to do. My biggest fear was that Star Wars would be so huge, people would be unable to see me in anything else." But McGregor is and established talent- and with Trainspottings scallywag junkie Mark Renton so deeply etched on moviegoers' memories, typecasting as a sci-fi hero seems unlikely. Just in case, his recent output has been wildly varied.

Forthcoming projects include Stephan Elliott's Eye of the Beholder, in which he plays a surveillance expert who stalks a female killer; and Rogue Trader, in which he plays Nick Leeson, whose financial derailment of banking's rich bastards he admires. He is also soon due in Australia to star filming a Baz Luhrmann musical alongside Nicole Kidman, Moulin Rouge. If McGregor has been seen as the kind of guy who does interviews in a sawdust boozer and ends them borrowing some cash and shooting his mouth about the impostors who are in acting for the money, this view will soon change. Even before Star Wars forces him into the deluxe celebrity compound, his largesse with his time and opinioins is already finding limits. The very famous can not afford to share themselves on demand; when we met he confided he had learned to beat back the tide of admirers that clustered nightly around the stage door while he appeared in Little Malcolm on a London stage recently. "If I tell them I have to go in and get on with my work, they thing I'm being uppity. I hate all that, 'Keep your feet on the ground son', stuff. You have to protect yourself….One night I'd just come off stage and this guy came up with two light-sabre toys and said: 'Can we have a fight?' He went on like I was being a prick, but what on earth was he thinking? I can't sit in a bar. I can't go to my local pub any more, unless I want to be hassled."

This is the exasperated do-me-a-favor tone he uses to savage famous names in print; a dangerous game for an ambitious actor, and one played in full knowledge it can only make life harder. He can not look at Jim Carrey, has slated Will Smith for Independence Day and he accused Minnie Driver of going to the opening of an envelope. "I don't give a f-what they think of me…" But this is not the anger of a bitter outsider, nor even fetching insolence.

McGregor is far too romantic a figure to truly unnerve us. Beneath the script, he is always as sweet as his terrible haircut and dubious knitwear. His dad tells him not to swear in interviews, and he tries hard; when he shivers from cold turkey in Trainspotting, or poverty in Little Malcolm, you want only to wrap him in a blanket and feed him soup. Being innately seductive was never going to be enough for an actor out to prove his range- perhaps his tough talk and larky put-downs are a way of asserting a darker edge. But what about sparing the feelings of those he dislikes without even knowing- for being, in essence, lesser people or artists than he considers himself to be?

He was embarrassed about slagging his old friend Minnie, he admits, and I won't mention it again, will I ? Before I can agree he has added, with suicidal alacrity; "In fact, what I said about her was true. I was right. I'll never change- I can't bite my tongue." We stare at each other over a running tape recorder. "Oh dear, have I just made another mistake? You won't put it in, will you?" Is he begging? "I'm begging. Please. Please." Even in a charmed life, some lessons have to be learned the hard way.