I do read P.D. James because she pays much more attention to character, to a particular atmosphere or setting. But most mystery writers, I think, are controlled by the plot.
I'll see something or hear something. Sometimes, it can be a color. Or a piece of music. Or an image of some kind. I see something, and it has huge emotional weight, although I have no idea why.
In Baltimore, I was walking with a friend who was playing at a pub he kept referring to as the Horse. But when I saw the sign 'The Horse You Came In On' - I thought, 'My God.' I had no intention of ever setting a Jury novel in the U.S., but when I saw that, I thought, 'That's it.' The names are very important.
There are people who read Tolstoy or Dostoevski who do not insist that their endings be happy or pleasant or, at least, not be depressing. But if you're writing mysteries - oh, no, you can't have an ending like that. It must be tidy.