Hey Storytellers,
The use of twists and big reveals in your games are a must. Of course, there is a right way and wrong way to do a big reveal. Harry Potter did this incorrectly, because there was really no foreshadowing for the readers about the Deathly Hallows and how all of a sudden these are somehow the most important thing in the story. Sigh.
Twists, on the other hand, can be done with no foreshadowing, because a twist is something small like your mentor is really using you to collect all of the things he needs to summon a demon. You thought you were gathering all of this to seal the demon away, but once your mentor begins to chant you realize he is trying to bring in the demon, not banish it forever.
Big reveals are different than twists, because they have greater, longer term impact. Sure you felt betrayed by your mentor, but in the end you killed him and sealed the demon away. Shifting your whole story to now center on the big reveal (cough Harry Potter cough) needs to be brought up many times in very subtle ways so that your players are surprised but not feeling like this has come out of no where.