Although any theatre company should pride itself on its unpredictability, the Oxford Stage Company, with which I work, has recently settled into something of a pattern, not created deliberately, but now we're almost rigid about it. For some reason, every play we do has two intervals. (Except for Billy Roche's The Wexford Trilogy, which, sculpted around five intermissions, must be something of a first. But given the amount of Guinness gulped on a good day, bladder decompression made this rather necessary.)

Two-interval evenings are now highly uncommon in the theatre. Many directors seem loath to have even one. When we began to try it, it was fascinating to watch the audience. During the first break, there would always be slight discomfort and unease. Should we stay or should we go? A drink now and an ice cream later, or an ice cream now and a drink later? Oh my god, how can I be expected to entertain my husband through two of these? It reinforced that strong sense you get sometimes that people go to the theatre not to be with each other. Michael Codron, the doyen of West End producers, once told me when we were thinking of bringing a gay play into the West End: "You have to be careful, the gay audience only lasts about three months, then there's nothing. What you really want is a play for couples who hate each other. Sad marrieds. They'll keep coming for years."

Yet by the time the second interval swings round, even in these frozen social times, people start to thaw and unwind, to talk about the play, about each other, or even to sit in happy silence. It's then that you begin to remember what theatre was always supposed to be: a social event, a great public get-together, where you took a good look at your fellow man or woman through the prism of the show. People have always come to the theatre to flirt, to politic, to talk, to traduce, to gossip, to fight, to face out social disgrace or to enjoy it. Whether it's Athens or Jacobean London, or 17th-century Paris, or late 19th-century Moscow, showtime is not just about what the actors do to the audience; it's more about what the audience do to each other. You sometimes get the impression, from the past, that the shows were a rather unnecessary distraction from the main event.

Chekhov's four-act plays would enjoy three long intervals. Forever absent from his own first nights, Chekhov would receive telegrams in Yalta about each act as if it were a completely separate event - "First Act, a triumph", "Second Act, disaster" and so on - and the actors would undermine the suspension of disbelief by taking curtain calls at the end of each act. Then the audience were off for half an hour of beluga caviar and discreet groping. This is what theatre is supposed to be, a rowdy, raucous, unifying social event, with the play as its focus, not as its tyrant.

I don't know how we have descended from that state of affairs to the sad shuffle of encircled souls that we have become, grouchily moping into a playhouse and treating anyone who tries to start a conversation as if they should be sectioned, but I'm sure it's time to reverse the trend. Part of the problem lies with the hegemony of directors, and the tricking up of theatre into an art form and a high-culture event. The great events that do still occur, the Royal Shakespeare Company's History season, or Tantalus, do create a form of this excitement, but it is largely expressed through an audience made up of theatreati, professors or Mini-Me versions of Michael Billington. That's all well and good, but it's hardly Shakespeare's swilling groundlings or Natasha and Prince Andrei, is it?

It is also directors who have done most damage to interval culture. It is they who enjoy the enforced agony of people having to sit in the same place for two hours. The unbroken spell, they call it. There's a form of defensive aggression behind this sort of incarceration. Not just a terror that the audience will leave (although that comes into it), but also a fear that if the audience have a chance to think, if the unrelenting intensity stops for a minute, then the whole house of cards will collapse. This is the neurotic insecurity of the nervous conversationalist, the one who won't pause for breath to let you speak, because she can't admit another point of view. Or the neurosis of the conman who has to maintain an illusion of charm and authority, and conceal the frantic paddling beneath the surface. Theatre should be one group of healthy individuals inviting another group of healthy individuals to enter into an illusion for the sake of the highest form of entertainment. Locking the audience in for two hours of imposed suffering is not a generous way to treat other people.

There is also a limit to what the mind can take. The human brain can only take so much music, thought, emotion, insight, humour, colour in one gulp. After that, you need a cigarette. If you don't get one, your concentration slips, and an evening starts to get long quickly. Chekhov wrote for his intervals. He knew that after 40 minutes of The Three Sisters, you need a break. If you string two acts together, you immediately get slow, boring Chekhov, because it's not what he intended. While you're grinding your way through the second act, Chekhov intended you to be downing a vodka or staring at somebody's cleavage.

But no matter who the writer, theatre is tense. It's intended to be a high-wire act. Actors can forget their lines, or fall off the stage, or have heart attacks and die. That, to be callous, is the fun of it. It is the essence of its liveness, that it is always on the point of collapse. It is one of the few arts whose joy lies in its potential for disintegrating in front of you. This is what the plate-spinning fraternity, with all their frantic falling over, do not realise. They are always strenuously underlining the liveness of theatre. It is already live. The audience knew that when they bought their tickets. They want to see something complex, difficult, beautiful and wise being brought off with speed, agility and grace. The achievement of that, that particular walk over the tightrope, is what makes theatre great. Being a crap mime, or a bad clown, and being proud of it because it proves how live you are doesn't quite hit the same level of endeavour.

And it is this fragility that makes the experiencing of it tense. Compare a picture of a cinema audience with a theatre audience and the difference is remarkable. In the cinema, people slouch, relax, lounge and reflect; in the theatre, they are alert, tense, febrile. That tension is right and proper - it makes an evening crackle - but there is only so much that spectators can take at a time before it becomes wearing. For that alertness to be maintained, they need ample opportunities for its contrary, a stiff drink.

Time is mysteriously elastic, and nowhere more so than in a theatre. No one knew that like Chekhov, who saw more clearly than most its potential for heavy length and for elusive brevity. It leaves me clueless, but of one thing I am certain. An excellent production of the The Cherry Orchard with no interval would last three hours and feel like a week; the same production with three intervals and good company would last five hours, and make you shiver at the speed with which it passed.

Dominic Dromgoole is artistic director of the Oxford Stage Company