Someone at my last talk asked if the crime comes first to my mind when a book idea is born or whether some other thing comes first and the crime follows. The answer? I don't know.
It's a process a bit like childbirth (sorry, guys). Once I've finished the book, the conception and gestation part gets kind of blurry. I forget the pain, and all I see is that beautiful baby.
I know I wanted Elizabeth Tudor in a series, so I guess character was the seed in that one. And my newest, just finished and NOW AVAILABLE, centers on a character who is quite unique, but the first thing I recall about it is that after living in Richmond for a short time I wanted to set a book there. So I think it's bits and pieces: a character I like, a plot that's intriguing, a setting of interest.
I do know that once I start on any book, I make a detailed outline of the crime itself, even if it does not enter into the book as such. No matter whether you're revealing a character, a setting, or a theme, a mystery writer has to understand the crime in order to lead the reader to it, giving fair clues so the reader may, if she wants, guess at the solution along with the protagonist.