Quirks are can help define a character, and often better than pages of description about eye colour, height and weight ever could. The main character in my WIP reminds herself to breath whenever things get intense. Another character has an iPod that he’s connected to like a lifeline.
For today’s prompt, imagine a character quirk and develop that, either for a secondary character in your book (that lacks a quirk) or for a new character. Who knows, it might lead you to a character that deserves their own story.