I write domestic suspense. The bad people in my books are philanderers and emotional abusers, financial manipulators, unreliable narrators with horrific back stories and, in general, people who play fast and loose with the rules. As a helicopter mom of two young kids, I'm kind of a stickler for rules. So how do I write these characters?
One way is through emotional substitution. I try to get into the feelings driving a character's actions by thinking of a time that I have felt similarly, albeit not to the same degree. Though I can't relate to the anger my character may be experiencing stalking her husband's girlfriend, I have had times when I've felt betrayed and angry in my life. I overlay these experiences to write my character's emotions in a believable way.
The danger of not using such substitution, I think, is that characters' actions can read false. I end up writing a lot of "he nodded" and "she grimaced" in scenes where the person would more likely be either standing dumbfounded or attempting a smile to cover her disgust. It's too easy to forget how I and other people actually behave in difficult situations without connecting to how I have actually behaved in my own life when under stress.
So, while I don't advocate going out and doing drugs in order to write a scene about a person out of control on drugs, I do recommend meditating about a time when life has felt out of control and then substituting in those very real actions and words when writing.