A quirk is a peculiar action, behavior, mannerism or personality blip that is generally unique to an individual. Everyone has at least one, even if they aren't aware of it. Celebrity quirks are often fascinating, like the president who kept a jar of jelly beans on his desk, the talk show hostess whose entire wardrobe is black, or the world champion golfer who has to carry three pennies with him every time he plays.
Quirks can add dimension and realism to fictional characters. A perfect character is basically cardboard, but one who whistles old show tunes, wears a black bowler or calls everyone he meets "pal" or "doll" demands more attention. The quirks themselves don't have to be huge things, either. Sue Grafton's P.I. protagonist Kinsey Milhone loves sandwiches with the repulsive-sounding combination of peanut butter and pickles (and she was quite put out in one novel when she discovered that a long-lost relative shared the same quirk.)
Writers themselves have plenty of quirks. One writer friend of mine has a pair of lucky writing socks, Lord help his wife if she ever loses them in the wash. Another writer I know always prints out the first copy of a manuscript she's written on her favorite pale blue paper. Other quirks I've heard writers admit to seem to me more like a method of mental/physical/environmental preparation, as in praticing yoga before every writing session, lighting specifically-scented candles near the work space or perching a new Beanie Baby on top of the computer monitor.
I have my fair share of quirks, too. Years ago I stopped buying clothing with prints or patterns; my wardrobe is now 100% solid colors. I keep books to read, jars of pens and blank notebooks in every room in the house, including the bathrooms. As for work, I never eat an hour before I write or write while barefoot. I write facing a blank wall with my back to the windows in the room. When I know for whatever reason that I'm going to have a tough writing day, I wear a really big, ugly old torn-up T-shirt I stole years ago from my guy. Even if I don't reveal it in the story, I always give every one of my characters a birthdate and a full name. Whenever I finish writing a novel, I never type "The End."
What are some of your quirks, writing-related or otherwise?
Jennifer Jensen's article on characterization and using quirks, Create Distinct Characters ~ Vary Personalities and Backgrounds for Interesting Fiction.
Author Judy Reeves has a entire page of interesting famous writer quirks here (I love the last one.)