Edinburgh Festival Day 9: Get it right next line: Jim White watches Mike McShane, Jim Sweeney and Steve Steen in 'rehearsals' for their improvised comedy show

This is how you rehearse an improvisation show. First you find a large room and half fill it with packets of chocolate biscuits. Then you scatter a couple of newspapers about the place, preferably ones with tortuously difficult crosswords. On the Walkman someone has brought along, you put a CD of Carl Stalling's music, written for the Warner Brothers cartoons, which has a whole booklet of sleeve notes which will take everyone at least 10 minutes to pore over.

The background scene thus readied, your improvisation troupe can stalk around the room saying, 'That's all, folks]' a lot. After five minutes of this, you then arrange for a demonstration by some local Muslims to pass by the rehearsal studio, so that everyone can adjourn to the open windows to make a few jokes about the guy on a bike at the back of the procession being, perhaps, not the most dedicated of demonstrators.

At least that's how Mike McShane, Jim Sweeney and Steve Steen were rehearsing last week for their show which begins next Monday at the Assembly Rooms. And they should know how to do it. Between them they have over 40 years' experience of making people laugh, without the benefit of a script.

Not that the threesome, who came to national prominence doing improvisations (usually on the subject of Clive Anderson's hairline) on the Channel 4 series Whose Line Is It Anyway, are entirely clueless about what might happen when they step out on stage. When they ask an audience for a few suggestions, for instance, there are some they can rely on.

'Toilets, sheep-shagging and vibrators,' revealed Steve Steen. 'They come up time after time.'

'Or you ask for a household object,' added Jim Sweeney. 'And some Nineties man will always shout out 'wife'.'

They would rather people didn't shout out these things, however, not because they are in any way prudish, but because they have exhausted every combination of gags about them.

'The problem with suggestions you've heard before is that you cannot improvise them in the same way twice,' said McShane. 'Say you get something you did brilliantly two weeks before; try and recreate and there'll be enough pieces missing from your memory to blow it completely. I've died doing that so many times, I've learnt just to put it aside and come up with something fresh.'

At least the trio can look forward to an audience. The first time Sweeney and Steen brought an improvised show to Edinburgh they were not fresh from an award-winning telly series. They were fresh out of school.

'When we got our first review in the Scotsman it was the literary equivalent of having your scrotum pulled over your head,' remembered Steen. 'It said, 'A better way to have spent 50p would have been to leave one's car in the car park beyond the allotted time limit.' '

'We're mature. We've put the review behind us,' said Sweeney. 'We can't even remember who wrote it. Actually, it was Allen Wright. That's Allen with two Ls.'

'When you get a review like that,' added Steen, 'you tend to find in subsequent performances there's not an awfully big audience to bounce ideas off.'

And ideas from their paying customers are what the threesome thrive on. Take the Raymond Chandler sketch which, in between bouts of beep-beep-perfect Road Runner impressions, the team was working on at rehearsal. In this Mike McShane tackles Philip Marlowe (he has been studying tapes of Robert Mitchum to see how it should be done) while Sweeney and Steen play the variety of bums and dames who people such stories (their research had been less specific, but seemed to revolve around absorbing the lunch menu at the nearby curry house).

'What happens is, I start a typical Chandler-esque monologue,' explained McShane. 'Then I ask the audience to fill in character traits, or plot possibilities. So if I start off saying, 'Louis was a strange man. Every time he spoke he . . .', I'll point at someone and ask them to fill in the missing word - it will probably be 'farted' - and take it in whatever direction they lead me.'

Over the past week they had been alternating in the role of the nightmare audience, setting each other sadistically impossible tasks. So, in this rehearsal, they decided it might be useful to base their thriller on ideas generated by an audience of two: one journalist and one publicist. When he stops the action and points his finger, McShane is an intimidating presence: the mouth goes dry as you struggle for suggestions which do not centre on sewage or sheep. The comfort of their audience is something they have learnt is important.

'We try to play at about 10 or 11 at night,' explained McShane, 'when they've been seeing shows all day, are well warmed up and are a bit tipsy. I'd hate to do impro at eight in the morning. And the last thing you want them to think is that they're coming to some audience participation show.'

'We also structure things so that we do a few scripted sketches to get the audience in the mood. And then we sort of map out what the evening will entail and what will be expected of them,' added Sweeney. 'They tend to really go for things in the second half.'

Last week, McShane found his tiny rehearsal audience guiding his Marlowe yarn in the direction of royal scandals. For 20 minutes, with Sweeney and Steen playing a variety of molls, pimps and financial advisers, he weaved an improbably entertaining path through toe-jobs and pool-side paparazzi. Through it all he slipped in Chandler-esque sounding gags which, in the manner of a Blue Peter presenter, he had prepared earlier - 'I like my coffee like I like my women: weak and bitter.' It was a fabulous performance, the kind of thing you might think worth preserving for a larger audience.

'No,' said a panting McShane afterwards. 'It's disposable, that's the pleasure. You never get bored.'

'In 17 years of improvisation, do you know how many sketches we've actually managed to extract and put together as scripts?' added Sweeney. 'One.'

After Edinburgh, from 4 September, The Really Unexpected Show goes on tour (071-704 1265).

(Photograph omitted)