I tend to think of description in a point/counterpoint sort of way. I will provide detailed description of some settings and then say almost nothing about other settings. My theory is that certain settings have greater relevance than others, almost as if they are characters themselves. When it comes to people, I provide as little description as possible. I always have in the back of my mind individual preferences about who someone would want to see playing the part in a movie. I want them to do the casting, but I need some level of control over the set.