There is a little girl I babysit that I like to call a magpie. She finds two, three or six toys she likes then carries them around all day. I have to pry them out of her clenched fists at meals and naps. I don't know what it is about each object that catches her attention. The other day it was two Uno cards. By the time she went home they were so crumpled and sweaty I threw them away.
I just read a book where one of the characters would cross her legs and jiggle her foot every time she was upset or angry. This wasn't mentioned every page or even every chapter. I noticed it four times, just enough to get a hint, a suggestion of her personality. Combined with a few other details we got the impression of a woman who didn't want to sit still, she was always moving, acting.
Subtle but interesting character habits, like the jiggling of a foot or the collecting of bits and pieces, can help round out a character, though one must be careful not to overdo it. If the author had mentioned the jiggling foot anymore it would be repetitive and boring. I would have been angry at the author for not trusting me to understand what they were saying. Pride and Prejudice comes to mind. Think of the description of Lizzy. What concrete things are we told about her appearance. Not much. Just that she had fine eyes and a couple of references to a graceful figure, but that was all we need. It was a lot more interesting to think about her fine eyes than for Jane Austen to say something like "She had wide open brown eyes that sparkled when she was happy." The subtle touch is much more satisfactory.
What character traits do your characters' have that makes them unique and interesting?