At university I attended a lecturer series given by a guest professor called Larry Beinhart on the subject of "how to write a crime novel." It wasn't part of my course, but it was one on the most useful things I went to. Two of his main points still stick in my mind:
1. The average novel never gets published, the average detective novel does
The reason for this is that there is a huge and hungry market for detective fiction, fans will easily read a novel a week. So they want more. Beinhart writes "hard core" crime novels, whereas I like the "soft core" variety (think Agatha Christie) but both types sell regardless.
2. Dialogue is action
I can't remember much more about this, I just remember this phrase.
I'm now thinking I should probably order one of his books.