"This is live theatre, missus, I'm not on the telly, you can't turn me off," says Ken Dodd, explaining at a stroke the difference between sharing a room with a performer and having him visit you on the flat screen in the corner.

But any member of an audience has the ultimate sanction on tap: the right to get up and leave. Apparently many did so during the recent run of The Marat/Sade at Stratford-upon-Avon, though not as many as was first claimed.

The people who get most het up about "controversial" or "immoral" performances are always those who have never seen them, the late self-appointed censor Mary Whitehouse being a star example.

In the early days of the Glasgow Citizens under Giles Havergal and Philip Prowse, when the productions acquired a not entirely inaccurate reputation for being louche and decadent (though an all-male Hamlet costumed entirely in black shot silk came some time after an all-male flower-power As You Like It at the National), the city's Lord Provost thundered that he didn't have to descend to a sewer in order to know that it stank.

Sometimes people like to make a great fuss about leaving. Actors in the first touring production of Joe Orton's What the Butler Saw became accustomed to the sound of furiously upturned red plush seats in the stalls.

And the Theatre Royal, Brighton, always a traditional out-of-town date before the West End, was the scene of many a middle-class protest against the new wave even as it washed right over them.

Nowadays people tend to leave without shouting at the stage. But I was surprised to find the house fairly denuded after the interval at yesterday's matinee of James Saunders's Next Time I'll Sing To You at the Orange Tree in Richmond.

One lady had remonstrated with the house manager at the interval that she simply couldn't stand it any more and was leaving. For good measure, and as a parting shot, she added "Utter tosh." Others just slipped quietly back into the suburban afternoon. But the college party in the balcony stayed put and apparently enthusiastic.

They've had full-frontal in-the-round nudity and the odd swear word over the years at the Orange Tree, and a fair amount of jovial blasphemy, but the audience never turns a hair. It was the still disconcerting, though dated, "theatricality" of the piece that was irritating some folk, its high flown philosophical content, its harping on existentialism and its unapologetic playfulness.

So you could argue that Saunders was still making his mark after all these years and that the cloak of amiability worn by the actors in the play is just that, a cloak. The subversive behaviour on view was far more mysterious and provocative than that of a sex show, or an Edward Bond play; the usual contract between actors and audience was being questioned with an uncomfortable persistence.

For some theatre-goers, things were simply not going to plan. Just as they didn't for anyone half way through the magnificent revival of Juno and the Paycock at the National Theatre on Wednesday night.

The door of the Dublin tenement apartment was opened to reveal the mourning party of Mrs Tancred; except that it wasn't. It stuck exactly where it was. Sinead Cusack gave it a bit of a kick. Other actors gave it a bit of a thump from the other side. Ciaran Hinds turned to his colleagues and, in character, asked if anyone had a key.

Then a stage manager with headphones walked on from stage left and said that the show would have to stop while they sorted the problem. Then the curtains came in. Disappointingly, when the curtains went out again a few minutes later, and the play resumed, the stage manager was nowhere to be seen. Having taken his place among the other characters so unexpectedly, it was surely his duty to remain for a little longer.

There are two points here. First, the incident was, unusually, not related to any technical system or failure; and doors normally let you down by opening when they shouldn't, or unexpectedly shedding a knob or handle.

And secondly, the momentum of the play was hardly altered, even though this occurred at one of the key mood changes in the narrative. It's as though the event ratified the "liveness" of the performance, and the audience felt doubly engaged for being witness to it.

The precariousness of theatre is its most exciting virtue, the fact that a set could fall down, an actor forget his lines, or an audience member cough so loudly that Ralph Richardson (as he frequently did) could ride the interruption like a skilled surfer.

Richardson it was, too, who once asked if there was a doctor in the house. Up stood a worried doctor, only for Richardson to say to him, "This is a ruddy awful play, don't you think, doctor?"

Despite a written text, a month of rehearsals, a detailed technical preparation, anything can happen and frequently does, and we never really complain about it. We actually love it.

I think it was in the first production of The Chalk Garden that a telephone rang at completely the wrong moment. And it kept ringing. The actors froze. Then, to their collective relief, they saw Sybil Thorndike move slowly, herocially, upstage and remove the offending handset from its cradle.

The ringing stopped. She put the phone to her ear and turned, proffering it to Wendy Hiller on the other side of the stage. "It's for you, dear," she said. But what happens next? That's the beauty of theatre. You never know.