The Art of Theatre: 7 Peripeteia: Nicholas Wright's Masterclass:

WITH a crash of gears the action goes into reverse. Tears are dried, smiles are abruptly wiped off faces, happiness turns to despair, defeat to glory. Peripeteia is abrupt reversal. It stems from radical news: a letter, a God, a telephone call. It strikes to the core of at least one character's life: no muted half-tones.

If more than one character is involved - and if the effect on each is different - then better still: 'This bond doth give thee no jot of blood,' declares Portia in The Merchant of Venice, plunging Shylock (who seconds ago was confidently sharpening his knife) into ruin. But Antonio, stripped for dissection on the other side of the stage, can put his shirt back on.

More see-sawing in Sweet Bird of Youth. Chance, the fading hunk, dreams of stardom for himself and Heavenly, his girlfriend of years back. And as long as Princess, the washed-up screen goddess, stays washed-up, he just might persuade her to give his career a boost.

But the success of her new movie knocks him brutally out of frame. A red-taloned carnivore once more, she's unfurling her leathery wings before she's even replaced the receiver. The last thing she needs is dumb reassurance - and she can pick up paid sex anywhere. Chance is out: 'Talk about a beach boy I picked up for pleasure, distraction from panic? Now? When the nightmare is over?'

She's civil enough to offer him a lift to the next town: his alternative is castration at the hands of outraged rednecks. But: 'That can't be done to me twice,' he replies. 'You did that to me this morning, here on this bed.' He stays.

Peripeteia is a heavy cannon, often passed over these days in favour of smarter weapons: wit, nuance, topicality. And it's true that plays devoid of peripeteia work fine, or more than fine, if well enough played and seen early enough in the run.

But plays which are tough and durable - which can, as it were, be woken at three in the morning, slapped around the face and still perform - depend on it. Shakespeare, Moliere and Ibsen used it all the time. The warhorses (The Importance of Being Earnest, She Stoops to Conquer, School for Scandal) would sink to their knees without it. So would every farce and every courtroom thriller.

High form or low, profound or silly, these are plays you can watch 18 months into the run, or 10 years later done by amateurs, or at an empty matinee, or in a vile translation or a German production which adds three hours to the running time: the play still shines through.

Even at its hammiest, peripeteia says something true about life - that nothing is safe or certain, and that it's precisely when you think you've got everything sewn up that love strikes out of the blue, or the doctor looks grave at the sight of that lump on your leg. I suspect that what it's really about is death.

Next week: Props

PRINCESS hesitates; then advances to the extended phone.

PRINCESS (in a low strident whisper): Sally? Sally? Is it really you, Sally? Yes, it's me, Alexandra. It's what's left of me, Sally. Oh, yes, I was there, but I only stayed a few minutes. Soon as they started laughing in the wrong places, I fled up the aisle and into the street screaming 'Taxi' - and never stopped running till now. No, I've talked to nobody, heard nothing, read nothing . . . just wanted - dark . . . What? You're just being kind.

CHANCE (as if to himself): Tell her that you've discovered a pair of new stars. Two of them.

PRINCESS: One moment, Sally, I'm - breathless]

CHANCE (gripping her arm): And lay it on thick. Tell her to break it tomorrow in her column, in all of her columns, and in her radio talks . . . that you've discovered a pair of young people who are the stars of tomorrow]

PRINCESS (to Chance): Go into the bathroom. Stick your head under cold water . . . Sally . . . Do you really think so? You're not just being nice, Sally, because of old times - Grown, did you say? My talent? In what way, Sally? More depth? More what, did you say? More power? Well, Sally, God bless you, dear Sally.

CHANCE: Cut the chatter. Talk about me and HEAVENLY]

PRINCESS: No, of course I didn't read the reviews. I told you, I flew, I flew. I flew as fast and fast as I could. Oh. Oh? Oh . . . How very sweet of you Sally. I don't even care if you're not altogether sincere in that statement, Sally. I think you know what the past fifteen years have been like, because I do have the - 'out-crying heart of an - artist' . . . Excuse me, Sally, I'm crying, and I don't have any Kleenex. Excuse me, Sally, I'm crying.

CHANCE (hissing behind her): Hey. Talk about me]

(She kicks CHANCE's leg).

PRINCESS: What's that, Sally? Do you really believe so? Who? For what part? Oh My God] . . . Oxygen, oxygen, quick]

CHANCE (seizing her by the hair and hissing): Me] Me] - You bitch]

PRINCESS: Sally? I'm too overwhelmed. Can I call you back later? Sally, I'll call back later . . . (She drops phone in a daze of rapture.) My picture has broken box-office records. In New York and LA]

SWEET BIRD OF YOUTH by Tennessee Williams, Act Three