Oedipus is someone you might have heard of. The basic gist: he was the foundling child who grew up to be the Greek king who found he'd killed his own Dad and married his Mum. The result: Oedipus poked out his own eyes and his wife/Mum hanged herself. Bummer. Even worse, it only all happened 'cos his Mum and Dad heard a prophecy that said their child would...Guess what: kill the Dad and marry the Mum, so they had a midwife put the baby out on a mountain to be exposed. Which she did and the child was found (or in some versions, the midwife raised the baby herself): the child then grew up not knowing his Mum and Dad were King and Queen of Greece... And the rest you know. How's that for dramatic irony, eh?
What makes the writer of Oedipus, Sophocles, great is the nature of consequence in his work. One event happens because of another: it is the very essence of drama, not only because of this "chain reaction" effect, but because so many of Sophocles' characters start with an URGE to do something that leads to their downfall. These are not passive protagonists who simply take what is thrown at them: they start off with something they need to do or find out and make decision upon decision that seal their fate. For Oedipus, he wanted to know who had killed his father. For another of Sophocle's protagonists, Antigone, she wanted to bury her brother against the wishes of her evil stepfather Creon. Orestes in Sophocles' Electra wanted to kill his mother and her lover for their murder of his own father Agamemnon, yet at the same time recognises that his half brothers and sisters (the children of Orestes' murderous mother and her lover) would in turn want to kill HIM someday for his own crime in killing THEIR parents.
It's this chain reaction effect however that makes Sophocles the king of reversals in my opinion. Of course screenwriting and the term "reversals" had not been invented back then, but Greeks had what they called "peripateia" which roughly translates to the same *sort of* thing, which is, also roughly:
Something that takes us from security to insecurity.
We're talking about Gwyneth Paltrow's character in Sliding Doors and everyone else in 90s rom-com land when they get sacked and find their partner in bed with someone else on the same day. We're talking about the big hero who is the big man - until he discovers he's out of his depth, whether it's John McClane stranded on the top of tower in Die Hard or Arnie in the middle of the jungle with an invisible alien in Predator. We're talking about those horrors or murder mysteries where we're asked to believe a certain someone is the threat or killer and it turns out it is actually someone/something else.
A reversal is, in essence, a plot twist that provides an obstacle that the protagonist needs to overcome.
One of my favourites? Has to be John McClane's lack of shoes in Die Hard. He's the barefoot warrior, but they're also his Achilles' Heel, for nasty Alan Rickman (can't remember the character's name) has the window glass shot out. You might say that "Ha! John McClane doesn't overcome his lack of shoes..." but he DOES for he is PREPARED TO WALK ON FREAKING GLASS BAREFOOT than concede defeat, showing us what mettle he really has.
All movies should have reversals I think... And those movies that keep us guessing as to what is going to happen next right to the very end have plenty of them. This doesn't mean that what is going to happen is a mystery, but rather the narrative is not pedestrian; instead it moves in ways we can't guess easily. They are simply not predictable, but narratives that are exciting, thought-provoking and/or challenging. All stuff we should want in our specs, natch.
Yet reversals do not work on their own. A plot twist becomes curious, out of sync, irrelevant, or just plain to dull compared with the rest of the action if it is not set up or paid off properly. Oedipus' discovery that he had killed his father would have lacked impact had he simply retraced his steps and said, "Oh Damn! You know what I've gone and done?" Instead, this successful, happy king decides to lay his one last curiosity to rest and asks a prophet instead who killed his father. When told it is he who is the criminal, Oedipus is deeply troubled and sets out to prove this prophecy wrong... Only to discover it was ANOTHER prophecy that is not only at the root of it all, but that if that prophecy had never been uttered, he would not be in the position he is now!
In other words then, a good reversal is related to good structure: you must set up in order to pay it off. You must promote the notion of that chain reaction; one event must happen because of another. Everything must relate to that "controlling idea" or central theme. You mustn't meander. Stakes must be high [whatever that means, not just literal life and death], the ante must be upped.
But a good reversal is also linked to good characterisation: you must have that active protagonist who will not rest until their mission is complete - even if that means their doom (literal or metaphorical). Other characters must give to that mission or risk obfuscating the protagonist's journey.
Easy, huh? I'll get my coat...
Oedipus Rex Synopsis
Glossary of Theatrical Terms [including "Peripateia"]
Useful Glossary of Screenwriting Terms [including "Reversal]
How To Write A Movie With An Unexpected Reversal
Review of An Unproduced Action Movie And How It Compares to Aramageddon In Terms of Reversals [phew]