I edit for a couple of murder mystery writers. What I like about this genre is that it has, at the same time, a fairly rigid set of requirements for structure, and a great deal of creative flexibility.
I recommend that all of the material characters are introduced as early as reasonably possible. I think it’s best to have at least three plausible suspects, but four or five can be a good number, too. I find that writers sometimes do such a detailed job of setting up the crime, that they forget to leave misdirection clues. Who else was at the scene of the crime? Who else had a strong motive? Who is the reader encouraged to dislike or mistrust? Don’t make it too easy! Readers love trying to solve the mystery like they love a good crossword puzzle.
Are you writing a murder mystery? I can provide proofreading and copy editing when you’re done writing. Or I can help you as your developmental editor–make the plot and character development stronger, more compelling. The better I know your work, the better I can help you brainstorm your next story.