When I first started my WIP I kind of had an idea but didn't really know where it was going. I had the general idea, but in my quest to figure out how to get it all down on paper, I realized I was eliminating any chance of a surprise, twist, unexpected event or even enjoyment. I was telling my reader all the stuff that since has been switched to be surprises, mysteries and puzzles that the reader and the MC get to try to figure out together.
That, I think, is one of the hardest things about being a writer - preventing the predictable. Sure, to some extent, there is some. Chick flicks usually end with the guy and the girl having that great kiss, hinting at happily ever after. Most action flicks have the good guy winning eventually. Granted, the cost may be VERY steep, but he usually wins.
What I'm talking about here is that point in a book where, even though you are reading silently, you verbally whisper, mumble, shout WHAT?!? and flip back through the pages to see if you really did read that right (Hunger Games, A Thousand Splendid Suns, The Mysterious Benedict Society, Harry Potter). It is those moments that keep the readers going, keep them wondering what is happening, keep them engaged.
I attended a conference for students once where the funny and brilliant Todd Petersen told the students that the inability to keep your reader guessing, in suspense, in the dark is the equivalent of giving them a crossword puzzle already filled out. It's not fun.
Readers want to be kept a little in the dark, to try to figure out what is going on, where the twist could come from. Readers want to be challenged, at all levels, even just a bit.
How do you prevent predictability? Without giving away spoilers, what books have shocked you with their twists, either about who a character really is, where the story is going, motivation, etc.