How long would it take them to realise their mistake? As they took their seats, surrounded by the kind of soap-wary young people they have read about in the Daily Mail? As the houselights went down and a shaven-headed youth ran on to the stage and yelled, at top volume, Graham Taylor's expletive of choice? Or as the boy began to spin his opening anecdote, a yarn which centred on him ending up drunk at a girlfriend's parents' house, so drunk, indeed, he soiled the sheets with every possible bodily function. Perhaps, by now, they'd spot their error as he continues explaining how, embarrassed, he bundled up the bedding into a stinking parcel and attempted to make an escape from the house (he will, he claims, return his haul later, nicely laundered). But his path was blocked by the girl's proprietorial mother, who insisted that if there is any washing to be done in her house, she'll be doing it. A tug-of-war ensued in which both parties lost control of the bundle. It arced ceiling-wards, before opening up and parachuting down, depositing the deposits within all over the girl's father, who was just tucking into his breakfast.
"Brown flecks of runny shite stained Mr Houston's glasses, face and white shirt," the boy recounts. "It sprayed across the linoleum table and his food, like he had made a mess with watery chip-shop sauce."
Well, let's hope the Mousetrap party twigged by then. Because from there onwards Trainspotting becomes really unappetising.
"When I came to deciding how to open the play," says Harry Gibson, who adapted the novel for the stage, "I felt I had to choose an episode which was light enough to draw the audience in. You had to start with the purely funny stuff."
There is a sense, though, in which Trainspotting shares one of the values of its neighbour: it is all about escapism. But while The Mousetrap's escapism involves disappearing from the cares of the world for two hours, Trainspotting is the kind of escapism associated with a particularly virulent boil being lanced: a great rush of putrescence which you can't help feeling is better out than in.
Indeed, since Welsh's novel was first published in the summer of 1993 it has exploded, as he might put it, like a squeezed plooky across the mirror of public consciousness. Everyone who has read it (and many who haven't) has taken their own spin on it. Tory councillors in Edinburgh claim its account of life there is a vindictively inaccurate vision of the fair city they sell abroad; New Labour councillors say that the book is right in that folk have suffered in the town under the Tory Government, though perhaps Welsh goes over the top since it is still, overall, a fair city which tourists are more than welcome to visit, provided they don't go to anywhere Welsh writes about. A critic in the Sunday Times opines, salivatingly, that "Welsh writes with a skill, wit and compassion that amounts to genius"; a week later, one of the critic's colleagues suggests in the same paper that Welsh is all hot air: "Working-class violence has an immense appeal to middle-class critics who couldn't punch their way out of a paperback." Across the country, students quote whole chunks of the book at each other as they used to the complete works of Newman and Baddiel; school children secretly exchange battered copies of the book of which they know their parents would disapprove; and the lads of Leith threaten that should Welsh ever return to their manor, he's due a good kicking for stealing all their stories and not giving them any of the subsequent poppy, hireys, or any other local term for money. Meanwhile Welsh himself remains cool and distant, sitting in exile in Amsterdam, saying little, getting his kicks from watching everyone get on with it. As he always does.
So to stage the book is to take a risk. Everyone coming to the play will have their own, fiercely personal, idea of what the thing is about. Harry Gibson, though, wasn't worried.
"The book comes from the great Scottish story-telling tradition," he says. "It is basically a collection of pub yarns, brilliantly realised. But told in a language which is so alive, frankly, all you have to do is put that writing on stage."
Which is what he does, lifting his script unaltered from the pages of the book, limiting his adapting work to editing down the picaresque plot into a form that fits the physical, and economic, constraints of the stage. The language, then, comes from the book fully, faithfully, intact. And it is this language, as much as the contents of the tales, that disturbs. It is a language stuffed with the kind of extreme expletives generally reserved for moments of intense anger or sexual passion (cunt, Mrs Whitehouse will be exercised to know, crops up 137 times in the play). But the violent, drug-fuelled, alcohol-wrecked lives Welsh's characters live ("their life is not just a little bit worse than yours or mine," says Harry Gibson, "it's on another planet") is so intense, such words are the only satisfactory ones for their everyday utterances. Welsh's cunning, however, is that though his lexicon is limited, his rhythm and tone is so fresh it never dulls: the language flies off the page with such velocity, readers can't help ducking. Reading the book is a bit like an encounter with the Tango Man: you get slapped about good and proper.
There is, for instance, a point in the stage play when the female character roars out "cunt" nine times in a climactic yell of defeat and pain. It is a moment almost as difficult to watch as the episode in which a junkie, running out of veins as rapidly as he is running out of options, injects himself in the penis. Well, perhaps not quite.
Language runs from Welsh's characters' mouth like a bad case of diarrhoea. An image, incidentally, that crops up regularly - as it were - in his work. In a diet as unhealthy as his characters, any lavatorial experience (and there are lots of them) tends to be uncomfortable, unhappy and smelly. An apt metaphor for their lives. And he is quite shameless, too: unworried by his reputation or the niceties of convention, he goes far further into the blackness than most of us would dare contemplate and, in a couple of cases which probably shouldn't be rehearsed here, too far for his adaptors to follow.
But the fundamental reason why the book works as a play (and as a film - the screen version will be in cinemas from February) is that the episodic yarns are so perfectly realised, fully resolved novellas, yet all linked by Welsh's mad imagination.
"It felt," says Harry Gibson, "like a bunch of scripts, ready and waiting. It is an amazing pack of stuff that Welsh has given us all to pass round. It will have an amazing energy in any media. It will make an incredible T-shirt."
n 'Trainspotting' is at the Ambassadors Theatre, London WC1 to 27 Jan. Booking: 0171-836 6111