The queue snakes around the bar, stretches down the corridor and up the tightly packed lane to Edinburgh’s Victoria Street. It is first come, first served and nobody wants to miss out. As the audience rush into the main space of the Liquid Room’s Warehouse, the mood is fevered. The stage is empty but for a rocking chair, yet people are taking selfies in front of it. The conversation is loud and animated. Someone is blowing bubbles. The one-off event is the definition of fringe cult. We are here for the 15th annual performance of A Young Man Dressed as a Gorilla Dressed as an Old Man Sits Rocking in a Rocking Chair for Fifty-Six Minutes and Then Leaves ... All you need to know about the show is in the title. The atmosphere is something else. Imagine a cross between Mornington Crescent on BBC Radio 4’s I’m Sorry I Haven’t a Clue and The Rocky Horror Picture Show: a game in which the rules are opaque and audience participation is fundamental. It is as if the Dada movement has gone mainstream. To ear-splitting applause, on comes the gorilla from the back of the room. Dressed in tweed jacket, glasses and cap, he settles into the rocking chair with his pipe at the ready. The audience shush themselves and then roar with laughter: their own reverence is funny. So little happens over the next hour that the smallest movement becomes momentous. That is the joke. It is comedy Butoh. The chair rocks back, then rocks some more, the audience gasp in anticipation, the chair rocks forward and the audience cheer. Two or three times this process builds enough tension to win a tumultuous standing ovation. With patience, there are variations. After 20 minutes, the gorilla slowly raises his pipe to his mouth, generating hysteria in the room. “Show some respect,” shouts someone. “Pushing at the boundaries,” says another. Similar moments of ecstasy follow when the gorilla scratches his ear, beats his chest, nods his head or raises a knee. But the performance – and the pleasure – is as much offstage as on. A slow handclap is shouted down. Phone torches are held aloft. Terrace chants come and go. A man is booed for going to the bar. Somewhere past the midway point half a dozen people place bananas on the stage. Someone dares to replace the gorilla’s glasses with some shades, prompting a spontaneous chant of: “Let him see!” “Do your old stuff,” is the night’s best heckle. “Be careful,” a close second. Despite calls for “one more song”, the gorilla leaves without climax or fanfare at 56 minutes precisely.