To believe or not to believe that is the question. ~ Albert Ellis
Ellis provided psychology further ideas about how thinking precedes feeling and action. What does this mean if the thinking is crooked? Crooked thinking can lead to irrational beliefs and these can direct odd behaviors that become automatic. Think superstition. Think quirks.
In the realm of psychotherapy one goal is to dispute these beliefs and to “straighten” out the thoughts. In the realm of character development it can create flaws, depth, areas for growth, interpersonal conflict, humor, and myriad other characteristics — much like it does in non-literary life. The author though gets to determine the significance of these beliefs and whether they get better or worse.
A character could have any kind of superstitions; odd or predictable. Scared of black cats. Sure. Goes to great lengths to avoid ladders. OK. Believes people with blue eyes are better than those with brown eyes. It happened in Iowa once upon a time. Maybe people with the name Fred are untrustworthy or those with the name Sarah are government spies. Let your imagination run wherever you want. Mine is going to the macabre. Serial killer cannibal mom who believes private school children taste batter due their leisurely lifestyles.
The beliefs we hold about how the world works are what determines who we are, to a degree, and provides a way to predict how we will act. If you know a character’s beliefs you can determine what they will do in a variety of situations. Knowing what they believe can help develop backstory too. The beliefs had to come from somewhere after all. It may be a way to explain the horrific or to soften the reader’s opinion of a cruel, cold character.
Being locked in the upstairs closet every time there was company coming over could lead to a lot of issues later in life and to a socially awkward adult. So would the belief that one’s nose is too big. What happens to the person who is told they will never be a success because of crooked teeth or because one ear is bigger or lower, or any other thing a person may have inadvertently told that person that for some reason stuck? They could be shy, mean, self-degrading, comedic, determined, or whatever you choose.
So the thing is, you can push the envelope of human expression and interaction if you give characters beliefs that drive all those individual quirks.