Are you writing about someone with strong traits, a personality disorder, or mental illness? Need a little guidance in getting their quirks or misdeeds just right?
I highly recommend this book by author and Ph.D. Linda N. Edelstein, who goes in depth with behaviors and the backgrounds which may have led to them. The hot topic for me — in life and in fiction — is Narcissistic Personality Disorder, and there is (to my delight!) much cross-referencing to be found within these covers on the subject. See all those paperclips in the photo? Bookmarks.
Is your character an addictive gambler? A shy introvert? Does she break the law, or oblige all the rules? Are there physical and/or psychosomatic symptoms? A history of abuse? Is there passive-aggressiveness? An overt and consistent cheeriness? Are there normal traditions, or rituals of concern? Name something of the human condition, I bet it’s in here.
Read storied examples, learn how to flesh out your character and his arc, even pick your antagonist’s career based on Dr. Edelstein’s expert insights. As one who writes AND has a fascination with the psychology of people, I can’t get enough.