What do characters look like?

I'm surprised at how little thought I give to what the people in my stories look like. I always include hair color, eye color, and basic build, plus maybe a dimple or a scar or a slight limp so the reader make them look however they want. This is the reason I'm happiest when my books don't have people on their covers, or at least, as in the case of
One More Summer, people but no faces.

Realizing this made me wonder how I could know the people so completely without being able to see them comple...