Backstage at London's Playhouse Theatre, currently showing Vesturport and Young Vic's Romeo and Juliet, a curious scene unfolds. Vanessa Redgrave is leading a group of unknown Icelandic actors in a tuneful rendition of "When You're Smiling". She is posing for photos with proud cast members, but it is she who is petitioning them. "It was so super," she beams. "Can I be in the show again? I'll be banging at the stage door to be let in."

This is no routine Romeo. It is an aerial acrobatics extravaganza in which Mercutio trapezes across the stage, Romeo dangles vertiginously from a vertical sash, and the actors free-style with the text, breaking into Icelandic whenever emotions bubble over. It may not sound like a show in which one would find one of Britain's most revered actresses. Yet Redgrave is not the only one to make a cameo appearance. Turn up at the Playhouse over the next three weeks or so and you could well see Sean Connery, Joanna Lumley, Jonathan Pryce, Dawn French, Derek Jacobi, Jenny Seagrove or another celebrity speaking the six-line epilogue. (The night I was there, so was Alan Rickman, but there is no date available to accommodate him).

"It's amazing," says Gisli Orn Gardarsson, who directs and plays Romeo. "Doing Shakespeare in English in the West End is incredible enough, but to have all these great actors on stage with us - we never expected anything like this."

Back in 2002, Gardarsson gathered some friends together to rehearse the play for six months in Iceland. They had no money for pay or sets, and scrounged for props. The trapezes were built by a blacksmith who lives in Gardarsson's basement flat; a springboard was found in a skip.

The director was determined to find a physical language for love. "Love is a concept that's hard to grasp only with words," he says. "When you're in love you feel like you can fly or do somersaults. I wanted to see how far we could take this. So we picked Romeo because Shakespeare's language is so poetic. It matches the physical action of flying around in the air."

After the Reykjavik opening, David Lan, artistic director of London's Young Vic, was so impressed that he imported the production in 2003, to great success. The show's return engagement is proving a harder sell. Not only is the out-of-the-way Thames-side Playhouse notoriously difficult to fill but the old-fashioned proscenium theatre is also not the kind of place to draw the cult crowd this Romeo needs.

Help came in the form of the president of Iceland's wife, Dorrit Moussaieff, who rang her friends Joanna Lumley and Jenny Seagrove. "I was very happy to help," says Seagrove, "It's a smash ing, irreverent, funny, sexy show. Joanna and I were both delighted to do it."

Word was out that these Shakespearean lovers desperately needed some star-crossing, and plenty of high-profile fans have signed up. "It's one of the most inventive stage productions I've seen for a long time," says Jonathan Pryce, who was recruited by Lumley. "It's incredibly well-spoken and intelligent. And it feels gimmick-free: the movement and tumbling reinforce what they're saying."

Pryce compares the show to the famous experimental spirit of the Liverpool Everyman in the 1970s. Seagrove reaches further back: "It's got the spirit, a sort of accessible, non-vulgar ribaldry that I would have expected at the Globe."

The production hopes to repeat the trick cleverly played by the hit 2001 Morecambe and Wise tribute, The Play What I Wrote, where every night a celebrity was wheeled out to be ridiculed and to boost ticket sales. But that was a comedy. This Romeo, for all its foolery, turns deadly serious by the end. Is there not a danger that the sight of a familiar face will jolt audiences into Hello magazine territory at a crucial moment?

"It could be a good or bad thing," agrees Pryce, "The ending is incredibly moving and you hope you won't break the tension. But it's swings and roundabouts. That theatre is so diffi-cult, they need to get people in there to see it."

"Of course there is marketing value," says Gardarsson, "but so far the stars who've appeared have made a positive difference. When Joanna Lumley walked on, for instance, she has such a presence that she really added to the moment. The audience went really quiet. As Romeo, I'm hanging upside down dead at that moment - but I still felt it."

Vanessa Redgrave's appearance also lifted the evening. Led by the clownish master of ceremonies, limping slightly after a hip injury, she spoke the lines mournfully, gazing in horror at the corpses swinging from the ceiling around her. It was as if a real person had suddenly been allowed to enter the show's crazy world and, through her, we could feel its pain. It is hard to imagine Dawn French having the same effect. But we can be sure that her fans will go along to see.

'Romeo and Juliet' is at the Playhouse Theatre, London WC2. Tel 0870 060 6631