In the midst of the story, the main character could suddenly discover something that changes the whole game. A good guy could turn out bad. Goals could be changed to a more imminent danger that arises. A character's internal conflict could lead to a shocking action. There are many ways to add in twists and turns to build suspense and keep a reader turning pages. Most novels will have at least one, if not a few, surprise moments. Whether it's a huge AHA! or more of a simple revelation, surprises are something readers enjoy and writers struggle to pull off.
Most surprises need to have some roots grounded early in the book--little whispers that could hint at something to come. Things that the reader might not consciously pick up on at first, but once the surprise is discovered, they can connect all the dots together.
Rachelle Gardner has a blog post that distinguishes the difference between foreshadowing and telegraphing, which is very important for working the element of surprise. Telegraphing (giving away too much, too soon) can kill the suspense and ultimately ruin the surprise moment, while some well thought out foreshadowing can heighten tension and keep a reader enthralled.
The biggest problem writers suffer when working in a surprise, is that you as the writer already know what will happen. And if you're revising/editing, you've been working on these elements for a while and might not be able to recognize if it's working. So how do you know if you've been successful?
Make a list of any and all foreshadowing or clues you've laid out. Go through each one and ask yourself, would any of these be too obvious? As a reader, would you have been able to figure it out before it actually happened based on these clues? And as always, your best bet--good beta readers to give you solid opinions.
If you're revising, how do you determine if you've done a good job of keeping the reader on their toes and making sure your surprise work? Leave any comments/suggestions below on how you work in your surprises!