Good Guy Turns Out to be a Bad Guy

The easiest possible twist to pull off (just behind It Was All A Dream and, the classic, Rocks Fall And Everyone Dies) because you can pull it off at any particular point and only need the explanation of, "Well they're EEEEEEEVIILLLLL!!" It's very often a crutch a bad movie pulls out of its ass in order to cheaply shock the audience. Bad movies will give no explanation for this (or a very, very poor one) and use it as an artificial excuse to raise the stakes and give some weight to The Final Showdown.

A good movie uses this to genuinely shock the audience. The film will have been building up to it the whole way through by dropping little clues throughout and making it be someone you least expect. The reveal should still be a huge blindside to the audience whilst, at the same time, making sense and explaining away some of the very suspect behaviour the character had been doing up to that point. This is extremely tricky to pull off but, done right, it can create a perfect and logical twist (Don Self from Prison Break, Nina Myers from 24 and Lotso from Toy Story 3 are this author's 3 best examples).

Of course, it doesn't really help when you cast someone who typically plays evil characters in this role. The more movie-savvy audience members will smell them from a mile away and not be in the least bit shocked when you reveal them to be absolutely correct about 15 minutes before the movie is due to end. (Note To Movie Producers: Dominic West always plays the bad guy or an arsehole. Always. Stop trying to shock us when it turns out his perfectly nice seeming character is actually an evil b**tard. Please.)