We all use them to protect ourselves when reality gets "real". Problem is, they're easy to locate in others -- just read the personal sniping in the comments section of any political article. But they're hard to see in ourselves. Since seeing oneself objectively takes hard work.
Perhaps that explains why writers seldom employ defense mechanisms in their characters. Since when behind a point-of-view character's mind, we identify with them. And thus, find fessing-up to having blind-spots makes us uncomfortable? Who knows.
Regardless, when characters use the same defense mechanisms we all do, they become believable. Not despite, but because of their contradictions. And once learned, defense mechanisms are easy to deploy in your work. And once deployed, they add depth with scant effort.
Fiction Writer's Guide to the 7 "Primitive" Defense Mechanisms
1) DenialWe deny reality to avoid the pain of having our preconceived notions shattered. We may be so afraid of something harmful, and thus ignore it.
To illustrate, consider this line uttered in the emergency room by a patient suffering from cirrhosis of the liver. “Do I drink? You mean alcohol, like an drunk? Hell, no. I only drink beer.”
2) RegressionAt age 36, actor Sean Penn found a paparazzi photographer hiding in his hotel room. A healthy adult in their mid-30’s would call security to remove the man, and perhaps press charges. Penn, however, acted like an adolescent, dangling the man from his 9th story balcony.
That’s regression. When facing a stressful event, we “regress” to an earlier stage of development.