Ever read that character who slams the table every time she’s upset or raising an eyebrow when he’s interested. Tics and mannerisms can add flavor to characters, yet they’re also the providence of the cliché. Nothing’s worse than using a worn out and stale tic for your character.
They key is to use these tics sparingly and logically. The logic comes from assigning specific tics to different emotional states for your character. Does she chew her fingernails when she’s worried? What about irritated or frightened. You could pick three or four key emotional states and then assign a tic.
Be consistent. Don’t mix them up, or the reader will get confused. You need to be like those continuity experts they have on films. You know, the one that knows precisely where to set the coffee cup for each take? Because mannerisms are like that coffee cup. No one notices when the cup is where it’s supposed to be. We only notice when it’s moved suddenly or is missing.
Be sparing. Don’t overload your writing with tics. Your character is deeper than this. I work on the rule of having no more than one or two per scene/chapter. Also, never double up on a tic. If you use finger chewing once, stay away from it for a few dozen pages. These mannerisms work better as subconscious cues for the reader, not billboards.