With the Halloween season approaching, it seems to be the right time to learn some lessons from horror films. Most horror/slasher movies end up employing a whodunit mystery - there's a killer on the loose and he could very well be one of the main characters... but who? It's a familiar source of tension, and more often than not, the resolution of that mystery is unsatisfying.
One thing that really annoys me in any whodunit - whether it's a horror film or just a standard mystery - is when the writer/s don't play fair. I hate when a writer cheats in order to get a shocking ending. It's fair to give the audience clues that are misleading, it's fair to drop red herrings that have a decent explanation and it's fair to give the audience clues that they innocently misread or misinterpret. But when you show the audience something and then say - "Psych! That didn't happen!" that's when I get annoyed.
Earlier this year, I saw My Bloody Valentine 3-D, a slasher film that had really only one main attraction - the 3D visuals. The slasher in question is a masked character disguised as a miner. We never see his face, only the mask, and early on that hints are dropped that it's one of the main characters - in particularly, Jensen Ackles' character, who has returned to town for the first time in ten years, and might have reason for flying off the handle in a rage, considering the woman he loved is now with someone else.
So after enough suspicion has been thrown on Ackles' character, the film reaches a point where he has to head down to the mine. While there, the killer shows up and traps him inside a metal cage. Ackles' is forced to watch helplessly as the killer slaughters a few miner. When other miners come to investigate, the killer flees and the others find Ackles trapped in the cage. Though the others start to suspect him, we - the audience - clearly saw that he was trapped in there by the real killer and that he was immobilized during the murders. Thus, the killer has to be someone else, right?
In the third act, it's revealed that Tom has in fact been the killer all along and hasn't realized it. He's delusional and is suffering from some kind of split personality. We're shown the mine killings again and this time, we see Tom commit the murder and trap himself in the cage so he can be found prisoner when the others arrive. It's a blatant rewrite of what we saw on screen as it happened! It's an utter lie to the audience and the worst kind of cheating in writing.
You can't show your audience something and then say "It didn't happen that way." You can go back and show them that something else was going on at the same time as the events they saw, but it's cheating to go back and rewrite history. If I had been hired to rewrite My Bloody Valentine, my solution would have been to not show the initial murder scene from Ackles' character's point of view. I'd have shown him going into the mine, found a legitimate reason to follow another character, and then have that character be the one to discover Ackles trapped in the cage near the bodies. Then, I'd have Ackles' character describe the murders as he believed them to have happened, in flashback. Thus, we know what he thinks happened and that version isn't shown to the audience from the seemingly objective and omniscient perspective as in the earlier version.
The reason why The Sixth Sense is such a brilliant example of a twist ending is that all the clues are right in front of the audience and are just presented in a way where they are overlooked. There's never a moment where Bruce Willis actually talks to an adult and gets a response. If there had been, and the movie then tried to explain that this conversation only took place in Willis' head - that would be an example of cheating.
Don't cheat in a whodunit. Even if the audience can't articulate exactly why that cheating bothers them, they'll sense it on a subliminal level. Such bad writing can bring down an entire movie.