Here’s what I’ve learned: The funny thing about fiction is it’s only good if it reads like a true story.
One consequence of that observation is: Don’t bend your characters to fit your story. Bend your story to fit your characters.
In other words, have your characters behave naturally, true to their individual natures. If they do, your story will feel true. Be sure your characters should never act inauthentically to suit a pre-determined plot line. Truth is what matters. If you can find truth in your writing, you will engage people. The worst response a reader can have is when they shake their head and think “so and so would never do such and such.” So make your characters’ reactions, thoughts, dialog, and behavior believable and consistent with whom they are. Your book will be more honest if the plot evolves from the characters, not the other way around. Let them tell the story and that will lead the story along.
Of course, that doesn’t mean you can’t surprise readers. In fact, you should. Surprise keeps your story interesting. You must avoid predictability as much as you avoid inauthenticity. But your surprise must be one that, upon reflection, suits a particular character in a particular situation, a believable reaction for that character, while still unexpected.
Unbelievable surprises may make the reader drop the book and never pick it up again. Of Mice and Men, by Steinbeck, is a wonderful example of a believable surprise. The reader knows Lenny has a problem with managing his strength and understanding the consequences of using it. However, without spoiling the story for those who have not read this wonderful novel, Lenny’s actions are a surprise, but a believable one, and the story is unforgettable as a result.
To put it simply, characters should act in character… and still surprise you.