Heard the one about me and Christian?

Irish stand-up comic Brendan Dempsey had never been in a play - until he found himself in a West End hit alongside Christian …

Irish stand-up comic Brendan Dempsey had never been in a play - until he found himself in a West End hit alongside Christian Slater and Mackenzie Crook

Ian Coppinger, a fellow stand-up and the finest friend a man could wish for, called me on Friday, July 23rd. He was rehearsing One Flew Over The Cuckoo's Nest with Christian Slater, Mackenzie Crook, Frances Barber and a cast of comedians. Half of them had been in the 2003 Edinburgh Festival Fringe hit 12 Angry Men. This year they planned to go one better: to the West End of London.

After three weeks of rehearsals the director and the actor playing Chief Bromden left the production. Ian said he'd put my name forward for the role of the chief. I thought little of it except that it was nice of him to think of me. I figured that, with only seven days of rehearsals left, a new director was more likely to go with a tried-and-trusted stage actor.

I had no ambition to work in theatre. I doubted I could motivate myself to repeat the same words, with conviction, six nights a week for several months. I thought I lacked the discipline and the ability, and it was too poorly paid to contemplate anyway: for the time and effort they put in, the money actors get in Irish theatre is appalling. They do it because they love it; producers get fat off their dedication, and it's a disgrace.

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The evening after Ian's call the producer Nica Burns phoned, asking if I'd fly over and meet the director the next morning. I mumbled about being supposed to join my family for a barbecue. "We'll pay for your flights and accommodation, of course," Burns assured me, and I agreed, figuring I had nothing to lose but some marinated chicken.

I took the Heathrow Express to Paddington, where I was collected by the diminutive but indomitable Burns, driving an equally small but not to be trifled with convertible Jaguar XK8. "You're not at all what I was expecting," she cheerfully told me, throwing a map at me so I could direct her to the director's house. I kept my head down and my mouth shut as she filled me in - and, no, I'm not going to tell you what she told me. As the film producer Robert Evans says, there are three sides to every story: mine, yours and the truth. The truth is that I wasn't there when the director and actor left the production, and I was less interested in knowing why than I was in trying to fill some shoes.

After one wrong turn - I'd been reading the map upside down - we found the house and went to meet not one but two directors: Terry Johnson and Tamara Harvey. Johnson would direct the last week of rehearsals in London, before spending August directing another play. Harvey would take over while we were in Edinburgh, carrying on until the first week of previews, in September, when Johnson would return. Apparently, this kind of set-up sends actors running, but what did I know? I'd never done a play.

"You've what?" Terry said. "I've never done a play," I repeated. He sat back, considering this, then gave me the script. Chief Bromden was the first character on stage. It read: "There stands Chief Bromden, a bull-muscled Indian of six and a half feet." I had the six-and-a-half-feet thing down, but the bull-muscled-Indian bit would need a bit of work.

We talked about the play for the next few hours. Well, they talked. I listened, offering the odd generalisation or platitude while reaffirming my determination to do the part. It wasn't a question of whether I could, or how much I wanted to do it: I had to. At six feet six inches, there are very few parts written for actors my size. This was one of them. (I'm writing the rest.)

Terry took me up to his screening room, to show me a clip from Nicolas Roeg's film version of his play Insignificance. Will Sampson, who'd played Chief Bromden in Milos Forman's film of One Flew Over The Cuckoo's Nest, from 1975, had a cameo as a lift attendant. At the end of the piece he switched off the television, turned to me and asked what I thought about the accent.

For the first time I relaxed - and gave thanks that I'd never listened to the friends and family I'd exasperated over the years with my mimicry and funny voices as they pleaded with me to speak normally. I answered in as good an impersonation as I could, talking about the accent, how it comes from below the vocal cords and whose enunciation is made at the back of the mouth.

That was that. Terry said he'd like me to do the part. I said I'd like to let him. We all went downstairs, where he made a dubious cocktail using vodka and a viscous, pale-green liquid from a bottle claiming to be apple Sourz. It worked. After downing a few we went on to the flat of another cast member, who was having a barbecue. I thanked Ian, who'd stuck his neck out when other actors would have kept their heads down, and met the rest of the cast.

It's odd when you meet someone you recognise from the screen. It's definitely them, but what are they doing here? Christian Slater welcomed me with open handshakes as I got reacquainted with some old friends and started making new ones. Frances Barber told me not to worry, that they'd get me ready. I told her I wasn't worried (I was drunk): I was going to do it ready or not.

The first week flew. I looked forward to getting up at 8 a.m. Three weeks behind everyone, I hadn't a spare moment. Learning lines, blocking scenes with the directors, working with dialogue coaches, vocal coaches and a choreographer. It's amazing what you can do when you haven't a second to doubt yourself.

I had 10 days, and then I had fewer. Christian got chickenpox. Johnson Terry told us Nica Burns was writing the press release that nobody was going to believe - she ended up showing doubting journalists the doctor's reports - and that we had the weekend off. Nica was putting me up, so I got to hang out in her lovely big house, learning my lines and conducting experiments to gain a more American Indian hue. I'd been given some fake tan from the Body Shop, which doesn't test its products on animals. Given my translucent complexion, I figured I needed something that would blind a dog or two.

Another actor came in to read Christian Slater's part, McMurphy; we acted around him for a few days before heading up to Edinburgh. A week to the day after he'd got sick Christian returned, planning a run-through, only to collapse the next day with a secondary infection.

Nobody was sure what, if anything, would happen. Secondary infections can lead to complications - pneumonia, pleurisy. The Edinburgh run was under threat, which was tricky, as we'd already sold out. The producers cancelled the first four shows. Then Christian turned up at the end of rehearsals one day. Nica said we were doing a full run-through the following day, in costume . . . and she was bringing an audience in. Which is to say that the first full run-through with the lead actor would also be the first dress rehearsal, preview and opening, all in one. It had never been done before, of course.

On August 10th, in the Assembly Rooms, the lights dimmed, the music started and I went out. There's nothing like a full house staring at you, waiting to see what you'll say, to make you realise who you are. I was a big pastry-skinned Irishman with dyed hair and too much make-up, pretending to be an American Indian. It was my first play, and I was the first on. I began speaking. The accent held up. The volume was fine.

But wait. What's this? My right knee began to shake. Or was it wobbling? The left knee joined out of sympathy - or maybe symmetry. Who knows? It was the most surreal moment I've experienced. One half of my body was telling me I was as nervous as hell while the other blustered on, refusing to believe physical evidence to the contrary. It was adrenaline, pure and simple.

The rest of the play carried on in much the same vein. Christian was like a man possessed. In One Flew Over The Cuckoo's Nest McMurphy is the catalyst who sparks everyone else's torpid, latent characters into life, upsetting the metronomic efficiency of NurseRatched's sterile, humming ward. Without that energy there is no show, and there was Slater, sweating out the remainder of the virus that had flattened him for 10 days. He tore through the play, riding over any bumps, finding any props that weren't where he thought they'd be or just carrying on regardless. After two hours of mad pumping energy - there are no intervals at the fringe - he threw his fake-blood-, sweat- and tear-soaked T-shirt into a standing ovation - and into a potential bidding war on eBay.

The rest, as they say, is a matter of historical record. The reviews were very positive, I'm told. We transferred to London at the start of September for two weeks of previews and more standing ovations. One night, after our fifth preview, I saw Terry and Frances Barber in animated discussion, bemused expressions on their faces. I asked what was up.

"You really don't have a clue do you?" Johnson asked. They explained that this kind of thing just didn't happen. Standing ovations at sell-out previews were not the norm, and it could not just be put down to the pulling power of Slater and Crook, the play or curiosity at a cast of stand-up comics playing it straight in their West End début. We had a hit show on our hands.

We opened on September 16th. Battle-hardened by five weeks of rehearsals and performances, Terry and Tamara had done all they could, and they wished us good luck. Terry's card contained a quote from Heathcote Williams: "Anyone else's opinion of your work is none of your business."

Two days later, with another sheaf of healthy reviews, One Flew Over The Cuckoo's Nest had become the fastest play to take £1 million (€1.5 million) in advances at the box office. It's still packing them in as we start our extension, and some of the audience (though rarely all) stand at the end.

I've now met many of my acting heroes - Simon Callow, Derek Jacobi, Kathy Burke - and discussed the play over a few beers with Burt Reynolds, who directed a stage version with Martin Sheen and Will Sampson. I've bowed to Dennis Hopper and Alan Rickman, had drinks with Cindy Crawford and held my own in a wine-fuelled discussion about Don DeLillo with John Malkovich.

There's a karmic price, of course. I've also broken my rib and little toe, torn off a toenail and continue to get bruised by Christian. It's a physical play, and he's a very physical actor. When he grabs you, you stay grabbed.

I'm often asked what's he like?And I really don't know. He loves his job in the way people tend to when they're good at it. Straight up and to the point, there's not really a lot of side to Christian Slater. What you see is what you get: he has neither the interest nor the guile to be disingenuous. We see less of him than in Edinburgh, which is easy to understand once you meet his wife and kids, who are with him in London. He works hard and is always anxious that he hasn't given everything and could do more.

Me? As the rest of the cast sort out what they'll do next I'm still trying to figure out what just happened. I'll be home at the end of January and available for work, if anyone's interested. Our final night is on January 22nd, but then again it may not be the last performance. Discussions are under way with US television stations to broadcast it live from London on January 23rd, which would mean starting the play at 1 a.m. As you may have guessed, it's never been done before.

* One Flew Over The Cuckoo's Nest is at the Gielgud Theatre, London, until January 22nd. Not Only But Always, a film about Peter Cook written and directed by Terry Johnson, is on Channel 4 on December 30th.