DONALD CLARKEcelebrates a clever twist to the tale

There's a great moment in Billy Wilder's Witness for the Prosecutionwhen Charles Laughton, playing the ageing defence barrister, realises an astonishing truth concerning the murder case at the film's heart. "I suspected something," he says, discombobulated for the first time in this cobwebby classic. "But never that. Never that."

This is precisely the reaction audiences should have to a satisfactory twist in a movie. You know something peculiar is up, but, distracted by carefully placed red herrings, you fail to grasp the exact nature of the reversal. He’s dead? She’s a man? He’s a hallucination? How did they get that past me?

Such thoughts are prompted by events – about which we will be respectfully vague – in two recent films. About two-thirds of the way through this week's A Perfect Getaway, something happens that is altogether surprising. A little later into the action of Orphansomething even more startling occurs. Have we managed to keep all things secret that should remain so?

Both those surprises adhere to Dr Screenwriter's Four Basic principles of Twistology. In order to demonstrate these in action, we must reveal a successful twist from a popular film, so, if, by some extraordinary circumstance, you do not know what happens at the end of The Sixth Sense, then look away now.

The first axiom is utterly trivial: the twist should be hard to guess. This is so moronically obvious that it hardly needs to be said, but we must be rigorous in laying down our core beliefs. Okay, if you've read Ambrose Bierces's An Occurrence at Owl Creek Bridgeor you come to The Sixth Senselate, following the many films it has "inspired", then you may have guessed that Bruce Willis was a ghost, but at the time there were few such stories around, and the reversal sideswiped many intelligent cinemagoers.

Secondly, the twist should be fair to the punter. That is to say, any reasonably bright observer should, after a good sleep and eating enough brain-enhancing oily fish, have an outside chance at spotting the looming hairpin bend. Remember the scene where Willis asks Haley Joel Osment when he sees dead people and the tyke looks straight back at his questioner and says: “All the time!“? That’s fair.

Thirdly, the twist should be consistent with the facts as presented. If you watch the film a second time and the script proves to be packed with events that contradict the twist, then you are within your rights to send a mad, misspelled e-mail to the director.

The final point is subtler: the twist should significantly alter your perception of the characters and of most major incidents in the story. In The Sixth Sense, three main characters – Willis, Osment and Olivia Williams, who plays Willis's wife – seem like entirely different people when we know the secret. The effect is nicely satisfying.

Sound sensible? Well, if you disagree, don’t bother sending Screenwriter an e-mail. Why? Because he’s been dead these past 20 years. Heh! Heh!