One way to create a plot twist is to steer the reader to a certain ending or event and then change it completely to something unpredictable. You can also take a scene in the book and tweak it into another direction and see what happens.
Rachel Scheller from Writer's Digest says this about creating plot twists. Ask yourself the following:
"What do I need to change to create a more believable world for each separate twist I’m including?
How can I drop the gimmicks and depend more on the strength of the narrative to build my twist?
Will readers have to “put up with” the story that’s being told in anticipation of a twist ending, or will they enjoy it even more because of the twist? How can I improve the pre-twist story?
How can I make better use of the clues that prove the logic of the surface story to create the twist and bring more continuity to the story—but only after the twist is revealed?"
She also says that readers want their emotional investment to pay off. This means that the plot twist never can happen in a way that makes the reader feel fooled, betrayed or embarrassed. A good plot twist deepens, never worsen, a readers investment in your book.
What you're after is that regardless of how many reads your book, no one is going to spot your twist before it happens. You succeed with this by coming up with every single way an event can play out and then forget about them. Come up with several new events and toss those too. What you're left with is an unpredictable event.
Another way to create a plot twist is to spread out tiny, tiny clues to what's going to take place. Too small to be noticed right away and you do this by distracting the reader by mentioning the tiny clues while something else, big is happening. This means that he/she will be focused on something else. When the reader reaches your plot twist he/she should think, "Of course, this should happen. How didn't I see it coming!"
A common plot twist is to take a good character and turn him/her evil. Because this is an obvious plot twist it can be hard to discise it until you want the reader to find out. Besides, when the reader finds out that the protagonist (the main character) is evil, they're also going to want to know why they're evil. Therefore, you may need to explain this for a long period of time after your plot twist.
One plot twist that you can try is to kill a character that seems unable to die. He/she can, for example, seem unable to die because it's one of the main characters in the story or him/her have been a part of the story from the beginning. Or, the reader deems him/her safe because the character can have had a few - or many - near-death experiences and survived them. (SPOILER!) For example, is when one of the main characters in Robert Jordan's The Wheel of Time dies.
Two other plot twists are that your main character turns out to have killed one of the enemy's relatives, and the other is when it all seems peaceful you give your protagonist more to deal with. You can also kill off your main character. The main thing is that the reader feels that "of course this was going to happen" AFTER the event occurs. A plot twist has to be understandable.
What is your take on plot twists?
Remember to follow your dream!