We Believe What We Are Told First

If you want an easy hook or twist in your story, follow the antagonist first. Make them human. They get up in the morning, put their pants on one leg at a time, eat breakfast, and chat with their friends.

Then you make the antagonist meet the protagonist and we see that the decent guy we met first is actually a cruel, heartless monster. The audience is confused. I thought he was the good guy? Wait a minute, is the "bad guy" actually the good guy? It's so simple a technique that it's basically a formula.

Another good formula in the same vein is giving the protagonist information that leads him on his quest for the large majority of the story (e.g. if you can find the jewel in the heart of the temple, you can break the curse), only to find out that the information was inaccurate or misrepresented (e.g. the gem actually powers the antagonist's ultimate weapon, and the protagonist disarmed all the temple traps for him).

But how come this works? Why do we always fall for the same ploy time and time again (unless the story is poorly-told)? From my experience, it is that people will always believe whatever they are told first. We assume that the first character we meet is the protagonist. We assume that information given by a character who isn't too shady is the truth. We assume that when we're told a character has died, they're actually dead.

It is a simple technique, but a very useful one. Represent a lie as a genuine truth or show the truth coming from an untrustworthy character. The latter method is nice because it makes the audience think they are a step ahead. The most important thing to remember about doing this is that if you are too heavy-handed about it, the audience will realize that you are up to something. Use a light touch. Don't tell them that a guy seemed really honest and genuine; show them that he smiled warmly and kindly.