On my last post, I mentioned using dialects as a way of adding a bit of depth and color to a character. I've used another way of fleshing out the quirks of a character and this tactic nearly made delete the entire chapter.
OCD. People, in real life, have this. Characters, in fiction, can have this. It's a major pain to write, but I think in Purposed Chaos (no publisher yet) it pulled it off well. I have a character who has developed a problem with 1's, 4's, and 7's. His actions, sitting down with two cups of coffee, washing his hands twice, etc turned out to be easy to handle. His dialog, on the other hand, was painfully difficult. I couldn't let any of his dialog contain 1, 4, or 7 words or any combination of words that would end in 1, 4, or 7. So, no 11's, 14's or 17's. Teens in general were avoided because they start with a 1.
It made the character quirky and it served a purpose in the story, but it was rather difficult. If you have the patience for something like this and the eye to notice when you stray from the OCD, I highly recommend it.