When writing a thriller/mystery, where the killer’s identity is kept from the reader until the end, do you feel the killer should be included in “X” amount of scenes, so your readers don’t feel cheated? For example: building clues toward several different people throughout a story, and then pulling the killer from “left-field,” who up until this point hasn’t really been involved in too many scenes. In your experience, do readers feel cheated, or do they like the twist of the unknown?

First of all, as far as including an unidentified or unknown killer throughout the book, keeping him (or her) on the fringes and then revealing at the end that he was responsible for the crimes—but doing so in a way that is both surprising and satisfying to readers—is very difficult, but if you can pull it off, it’s also very satisfying.

There’s no rule-of-thumb specifically for how many scenes you will want to include him, but it tends to make it more satisfying to readers if he is present throughout the book. I try to introduce him in the story as early as possible, and then have him reappear several times so that when the reveal comes it isn’t out of left-field.

You must always satisfy the reader by giving him more than he expects. Never let them feel cheated. In one of the paradoxes of fiction (and this is especially true with mysteries and thrillers), readers want to predict how the story will end, but they want to be wrong—yet still satisfied.

To make the twist work, it must be inevitable (the only possible conclusion when it appears at the end of the story), unexpected (so that no one sees it coming), and a revelation that adds meaning to what precedes it in the story.

I reread my stories as I’m working on them, usually printing out the novel once a week or so and glancing it through from the first page, so that I can grab hold of the context and try to experience it as the reader does—asking myself what the reader is hoping for, expecting, and wondering about. Then, I put my motto into play and try to always give readers what they want or something better.

There are several types of twists, and one of them is as you describe—giving clues that point toward the guilt of several people. This creates uncertainty in the mind of the reader. Another type of twist is to make all the clues point toward one person so that readers will be sure it is him. In this case, you are striving for certainty in the mind of the reader. Both types of twists can be satisfying, as long as the logic of the story supports that ending.