Why is appearance important?
Some authors can get away with writing a novel and never describing the characters’ physical attributes once. Most authors can’t. While it’s all fine and dandy to let the reader’s imagination run wild, they will, at the very least, need a push in the right direction. The better you are able to describe your character, the more real they will become in the reader’s mind. A character’s appearance can tell you not only their strongest preferences, but also give you clues as to the details of their personal life.
When you’ve completed this exercise:
You now have within your power the ability to describe your character in specific detail, without over doing it. Be careful though, because you never want to include the entire appearance description in your story, at least not all at one time. Take bits and pieces and use them when appropriate. Fanfiction writers can get away with only using “the brunette” because their readers already know the characters, but your audience has no idea who these people are, or what they look like.
Remember: Appearance is about knowing but only showing when people would actually notice.