My high school drama teacher had a quirky habit: first he’d pull on his nose, and then he’d push on it. One day, in the midst of a discussion about mannerisms, he told us where this habit came from: As a child he’d grown anxious that when adults said he had a “cute little pug nose” they really meant “he looks like a pig.” So he’d begun pulling on it to make it longer. Then one day, his grandmother told him that if he didn’t stop tugging it his nose would end up looking like a sausage. That was even worse! But he couldn’t break the habit of pulling on it, so he adopted a second mannerism of “putting it back.” This two-part nose-fix had long since become a subconscious mannerism. Which is to say that he was (normally) unaware he was doing it.

Good writers imbue their characters, whether…