I recently came across a writing blog called "Hey, There's A Dead Guy in the Living Room: Mystery Publishing from Idea to Bookshelf". They had an excellent post on character quirks, the strange little habits and idiosyncrasies that we all have and our characters should have to make them seem real.
Quirks can tell a lot about a person, or may simply be another identifier so the reader can follow the action in a crowded scene. Quirks can even have some greater symbolism. In my Civil War horror novel one of my main characters is a fifty-year-old captain in the Enrolled Missouri Militia. Now anyone who has read up on the Civil War in Missouri knows this Union militia got their ass kicked on a regular basis. Well, this guy is no exception, especially in the beginning of the book. He has two main quirks--he loves riding and he gets lower back pain any time he rides for long periods of time. Getting old, you see. The war has worsened this condition because he has to be in the saddle most of every day. Never an ardent Unionist, our hero is now losing the enjoyment of his favorite activity because of a war he hates.
This isn't a major part of the plot or even particularly important to it, although it does put him in a foul mood much of the time. What it does do is give symbolic example of the weakness of the Union militia and highlight our hero's dislike of the conflict.
So when you're writing up characters, try to think of some quirks. It makes the reading, and the writing, a lot more fun.