Mannerisms are little things everyone does unconsciously such as biting your nails when nervous or a girl twisting her hair while in thought. They are ingrained in our behavior and very hard habits to break if we need to. Mannerisms are often learned from a caregiver or close friends. How many times have you seen your best friend and her mom do the same thing, whether it’s a quirk in the kitchen or something they do while they think? When was the last time you noticed your friend and his roommate do the same little thing? Have you ever noticed that both you and your siblings have a habit of tapping you feet while waiting or that you all have the same smile? Things like that can distinguish a character in a story or tell others that they are family or hang out too much. There are so many things to chose from for your character, just remember not to overdo it. The story may believably point out a few mannerisms but pointing out too many may be overdoing it. How many of you would notice a person having over a hundred mannerisms and comment on all of them (not counting significant others)? That would likely be overkill unless done just right.
However, mannerisms done right can add depth to a character and possibly add plot points to a story or change someone’s emotions towards them (either good or bad).