sex, drugs and rock'n'roll; Trainspotting achieves the impossible: a funny, stylish film about heroin. Ewan McGregor's charismatically disreputable performance is pivotal to its success
"take the best orgasm you've ever had, multiply it by 1,000, and you're still nowhere near it." So says Mark Renton, the central character in Danny Boyle's already-frenziedly-anticipated-and-it's-still-a-month- till-it-comes-out film version of Irvine Welsh's best-selling Caledonian junkie romp Trainspotting. And it's not the pleasure of a fried egg sandwich on a cold day that he's talking about, or the excitement of finding an unread newspaper left on a train, but the joy of injecting heroin.

Trainspotting's willingness to face up to the apparently self-evident fact that some people take drugs simply because they enjoy them will not endear it to everyone. There is no doubt, though, that this is the most engaging, original and resonant cinematic portrayal of delinquent British youth since Quadrophenia. It not only banishes the memory of such risible recent domestic exploitation efforts as Young Americans and Shopping, but also manages the apparently impossible - to make a film about junkiedom so stylish and funny as to beguile people who find the whole idea stunningly tedious.

Ewan McGregor's charismatically disreputable performance in the pivotal role of Mark Renton is one of the keys to Trainspotting's success. The McGregor giving interviews in the Groucho Club is barely recognisable as the soaking wet, crop-headed desperado who'll be looking down from the film posters for the next eight weeks or so. The jovial 24-year-old has put back on the two stone he lost for the role and grown out the crop into an easygoing bed-head/stubbly chin combination. But the most noticeable thing about him is that he talks a lot of sense for an actor.

"It's a funny thing, me doing interviews," McGregor points out, "because it doesn't really matter what I think about anything: I just enjoyed being in the film." That, of course, is not the way things work, and McGregor is visibly bracing himself for the high level of controversy that Trainspotting's even-handed approach to drug addiction is bound to stir up. "If someone's constantly telling you 'Don't do this, don't do that'," he argues, "especially as a kid - the first thing you want to do is go and do it. It's much more responsible to say 'It'll make you feel fantastic for a short while but then it will lead to this, this and this'."

Though it eschews grim documentary realism for a more humorous, sometimes almost surreal approach, Trainspotting pulls no punches in depicting the levels of moral and physical degradation to which the junkie might expect to stoop. (In one memorable sequence, McGregor dives head-first into the worst toilet in Scotland in search of an opium pessary). But one of the interesting things about the film is the way it shows that, while for some people their first taste of heroin is indeed a one-way ticket to despair and death, others - like Renton and his unsavoury friend Sick Boy - have the ability to drift in and out of drug-taking.

Drifting in and out, as McGregor seems admirably well aware, is exactly what actors do. "'I became a drug addict for six months in preparation for this film': that's such a load of bollocks," he snorts derisively. "If you did that it's because you wanted to be a drug addict - it's not going to help the film." The Trainspotting press kit does mention some research done among junkies at a Luxembourg railway station while working on the new Peter Greenaway film. "I didn't hang out with them," McGregor insists. "I just watched them from a distance. I'd never initiate myself into the group because that would be too embarrassing [Assumes stereotypical idiot actor voice] 'Hi, I'm going to play a drug addict, would you like to show me how to do it?'"

Much is made of the advisory role played by the ex-addicts of Edinburgh's Calton Athletic Recovery Club in the making of Trainspotting. Even in a formal setting, surely it must still be quite embarrassing to meet people who have been through terrible experiences on the basis that you're going to pretend to be like them? "We met a lot of people who had been drug addicts, but I didn't feel that we were using them because I don't think they felt we were. A lot of them had read the book and really liked it, and they knew that we were quite serious about wanting to make it into a good film."

They say you never recapture the thrill of your first recovery experience. McGregor was certainly deeply impressed by his first encounter with Calton Athletic. "The first time Danny Boyle took me up there, I heard this guy Eamonn - who ended up being our adviser - tell his drugs life story, and it was extraordinary. I'd never heard anything like it; I'd never felt anything like the atmosphere of support in the room; the giving of strength to each other, from these hard men and women, felt almost religious. Hearing how low they'd sunk certainly dispelled any ideas that I maybe had of any glamour involved with the taking of heroin, because I probably did have some ... I'm sure I did".

Why do those ideas persist in spite of all the evidence to the contrary? "It's something that's got an awful lot of kudos because it's taboo, because it's the big bad one. I mean, why is it worse to be injecting heroin into your arm than to be doing a line of coke in a toilet?" McGregor laughs, aware that this might be a controversial statement to make in the middle of Soho, notorious red light district of coke-crazed media whores. "I suppose socially it's to do with the needle - the idea of an implement putting things into your body makes it all seem very clinical and medical and that sets it apart".

There is a big - and for non-syringe fans, appropriately wince-inducing - intravenous close-up in Trainspotting. Is that Ewan's arm? "It is my arm, but moulded prosthetically and with a plastic pipe going into a little pool of blood underneath so you can see the pulse." Cast members were inured to the horrors of the injection by Eamonn's expert tuition. "It didn't seem to mean anything to him," says McGregor. "I think what means everything to you is what happens after you've put it in your arm - everything else is irrelevant."

The irritating thing about the Irvine Welsh phenomenon is the way some people seem to think the merest acquaintance with his vividly scabrous fictional creations somehow qualifies them as honorary roughnecks. McGregor has no such pretensions. His background was plainly a good deal more comfortable than Renton's but that does not compromise the integrity of the film. Rather, it enhances its thrilling sense of mobility.

Ewan McGregor grew up in the small and sedate Scottish town of Crieff. Inspired by the example of his uncle, Local Hero star Denis Lawson, he had decided he wanted to be an actor by the age of nine. "I remember throughout my childhood in the Seventies he [Lawson] used to come up and see us and he'd always look really different from other people I knew. He had flares on and sideburns and beads and a big sheepskin waistcoat and didn't wear any shoes, and I just wanted to be like him."

The only cast member to survive from the Trainspotting team's previous success, Shallow Grave, McGregor has understandable hopes of being "associated with their work in the same way that Martin Donovan is with Hal Hartley". That didn't stop him taking a major role in a new film adaptation of Jane Austen's Emma. "I got married between Trainspotting and Emma," he vouchsafes, still somewhat bewildered. "One minute I was lying on the floor with a syringe in my arm, then I got married, then I was standing in this trailer - I'd never had a big trailer before, it was quite nice - with a wig, and top hat and tails, and leather gloves on, and for a moment I thought 'I can't go from skinhead drug addict to ha-ha-ha curly wig acting.'"

But surely that's your job? McGregor smiles contentedly. "I would think so, yes."

8 'Trainspotting' opens on 23 February. Irvine Welsh interview, Review page 18