Many writers worry about how their people, especially family and friends, will react to their work. Will people assume it’s autobiographical? Will they find themselves in the story, and, if so, will they mind that the portrayal’s unflattering? Will they imagine you yourself have perpetrated the crimes of your worst villains or suffered the humiliations of your most abject characters? Or simply assume that you are a nose picker because you've described one so vividly? These problems are minor compared to the ones I would like to remind you of.
First, stop and think about the most disturbing thing that you've ever done, or the most shameful experience you've endured. It could be something actually criminal, or just something very private, or a childhood transgression or humiliation. If possible, go for something you wouldn't talk about with anyone, ever, something that feels utterly unpublishable to you, something that makes you sweat just to think about it.
Now, write for 30 minutes — or as long as you can stand it — about this topic.
Feel free to shred, delete or burn the result as soon as you're finished.
Here's the useful part: Plant this secret — or something inspired by the secret — at the core of a character. Just as you never talk about your secret, your character's secret never needs to be revealed for you to use it as a kind of emotional thermonuclear reactor, a black hole around which your character’s galaxy whirls. This invisible, gnawing secret can be used a driving force, an engine of narrative, and could just possibly lie at the root of all your character's irrational behavior — all without ever being mentioned.