What makes quirks so much fun is that they usually tell you something about the character. For example, many quirks that I use in my books seem to be associated with nervous or anxious behavior. I have one character that twitches and jerks a lot especially when he's uneasy, whereas another character (who's bilingual) switches back to his native tongue when very upset.
What I love about quirks is that it helps a character seem much more three dimensional. This is especially true when the character regularly demonstrates the behavior. As I mentioned in the earlier examples, two of my characters tend to react in certain ways when presented with stress. This becomes consistent throughout the books to the point that you begin to expect it in the same circumstances.
Of course, not all quirks have to be connected to emotions. A quirk could be someone not wanting the food on their plate to touch or refusing to drink out of a blue glass. One of my characters, for example, has a huge lime tree in his condo. He's obsessive about it and treats it almost like a child that needs constant care. That might also be considered a quirk. Then again...maybe that's just weird.
What are some of the quirks you've noticed with your favorite characters?