English audiences love improvised comedy, yet it is most likely a sharp-witted North American - such as Canadian Mike McShane - who satisfies our appetite for witty wordplay and good, ol'-fashioned cleverness

Mike McShane first moved into improvisation from straight drama when "I started getting bored holding a spear behind Cleopatra and watching her die. My friend Greg Proops invited me to join an improv group in San Francisco, so I gave it a shot. I'd previously just used other people's words. This was the first time I'd presented my own point of view, mixed it in with others', and created something fresh."

Now, several years later, the Canadian McShane is one of the best in the business, able to improv till the cows come home, on either side of the Atlantic. He's putting those skills to good use tomorrow night at Her Majesty's Theatre as part of a team also featuring Steve Frost, Josie Lawrence, Paul Merton, Neil Mullarkey, Lee Simpson, Andy Smart, Jim Sweeney and Richard Vranch. They are all performing at the Improthon '97, a benefit in aid of Freemantles School for Children with Special Educational Needs.

After many years as a star on Whose Line Is It Anyway?, McShane still gets an unrivalled buzz from improv. "It's the thrill of not knowing what's coming next, of being surprised by your fellow performers and trying to match them," he explains. "There's friendly competition, but once it gets in a groove, it's great. English audiences, in particular, enjoy the word- play, the cleverness, the ability to juggle, to be put on the spot and come through."

McShane is always aware, though, of the lurking dangers of a backlash against a popular format. "Both Britain and the US have rampant consumer cultures where you love something one day, and the next you think it's boring because it's been done," he reflects. "So you have to keep up with the challenge of people's appetites and continually refresh the form. The promise of improv is that there's going to be something different every time. It's still off the cuff - and there's always excitement in that. That's why it won't die. There's nothing like it live."

The Improthon '97 takes place at Her Majesty's Theatre, Haymarket, SW1 (0171-494 5054) tomorrow at 5pm