As I leapt to smash the imaginary tennis ball, I heard Joan Littlewood shout: "You missed it!"

We were in the Hope when Geoff pointed to a man with a jug of Hoegaarden. "Isn't that thingy from The Cops? Should say 'Hello'."

Something held me back. Although the man certainly looked like the fat cop from Tony Garnett's series, I felt uneasy about making his acquaintance. If it was true that Garnett regarded actors not as special beings but as workers whose job just happened to be carrying a storyline rather than a bag of plumbing tools, wouldn't we be violating that ethic by treating one of them like a celebrity? Only when Geoff looked unconvinced did I play my trump card. "Joan Littlewood couldn't bear to have her actors recognised," I announced. "I know that from personal experience."

It was only a small fabrication. I spent six months as a professional actor at Theatre Workshop in Stratford 36 years ago but I had little more than a walk-on part and only got to see Joan Littlewood when she turned up one morning and led an improvisation session.

The scene we were rehearsing involved the stars of the production, Tom Bell and Billie Whitelaw, but they were interrupted at one point by the arrival of two tennis players (John Junkin and myself). We were intended as comic relief, a middle-class counterpoint to the proletarian intensity of the main characters, and the playwright, Alun Owen, spent some time teaching us to speak in the pastiche of an upper-class accent favoured by Scousers anxious to deny their origins.

Joan wasn't interested in accents. What upset her was that we didn't look as though we'd just been playing tennis. For an hour she staged an impromptu tennis match on stage. There were no balls or rackets but every actor took their turn on court and mimed a vigorous series of exchanges. She kept a sharp lookout for anything histrionic. If we missed the ball completely then we had to simulate the complex way a genuine player would lose and then reassert their composure, rather than falling to the floor in mock agony. I remember leaping up and expertly smashing the imaginary ball into the corner of my opponent's court only to hear Joan shouting from the side of the stage, "You missed it. You missed it".

At the time I was only four months out of drama school and so consumed by the idea that my minor role was a mere stepping stone on the way to stardom that all this improvisation seemed redundant. Surely I didn't have to mime playing tennis for an hour in order to deliver a miserly 12 lines?

It was a fatal presumption. As soon as I stepped on stage on the opening night I felt I'd landed in a foreign country. All around me were people I'd come to regard as friends: Bell, Whitelaw, Brian Murphy and Murray Melvin. We'd been drinking together, had lots of laughs backstage. They'd made me feel at home. But all those hours they'd spent with Joan Littlewood had worked their magic. They were suddenly strangers to me, utterly absorbed by their new identities, absolutely content to surrender themselves to their theatrical character. So mesmerised was I by their transformation that my accent deserted me and I trampled carelessly over Junkin's best line.

I never acted professionally again. All I'd wanted had been personal recognition, public acknowledgement of my identity. My 15 minutes of terror-struck loneliness on the stage of Theatre Workshop taught me that the only acting properly worth admiring was that which remorselessly denied any such ambition.

I fancied the man from The Cops was on my side. When we next looked up he'd slipped quietly away into the night.