Who Done What?

The above question might just define the mystery genre.

Mysteries are a bit like romance novels, in that they, too have a formula. Someone did something that needs figuring out. It can be done so all the details are there for the reader to figure out, or it can be written so the reader probably won't figure it out before the Great Reveal even though the narrative character does.

The former type tends to be a bit more popular, if you ignore Sherlock Holmes. A well-done mystery is an exercise in logic.

Due to the logical puzzle nature of genre mysteries, that limits what the person uncovering the mystery can do to solve it and what the mystery itself can be. The mystery can't be solved without rational methods, and the culprit has to be believable without necessarily being obvious. (Exceptions make stories more thriller mystery than straight mystery.)

The crime, the culprit, and the person solving the mystery all must be introduced early in the story or book, in the first few chapters. That means you usually start off with several characters, so the reader doesn't necessarily know which of the side characters introduced is going to be the baddie.

Violence is also the rule of the day in genre mystery, preferably murder. Other suitably violent topics to warrant a novel about solving them are still generally considered taboo, like animal cruelty, child molestation, rape,

With a mystery, rational structure is key. You won't want 'clever' plot twists and improbable events—what you want is a logical plot progression that guides the reader through all the details he needs to know what's going on, without the reader likely figuring it out. Don't add disguises that wouldn't work, random twins, accidents that solve everything, paranormal things that change everything, a culprit who's actually your lead good guy.

Anything weird or "different" that you add changes the genre. So if you want to write a genre mystery, shelve the tweaks for when you want to write suspense or a mixed genre.

Which means you have to do your research, just like you should in any other genre. That college-aged chick you get to read your book might've taken a forensics elective or done some independent study. (Who, me?)

Fingerprints aren't easy to lift, and people don't carry guns barrel-up. It's barrel-down for a good reason. Namely physics. If you mess up something in your book, people will notice. If it's a big enough something, those people will spread the word. Which will hurt your sales and your reputation as an author.

Now, there are subgenres within the mystery genre. So if you want to write a mystery and not worry about it being strict in the genre, ask yourself the question: "Who did what?"

Make the what compelling enough, and the who intriguing enough, and you'll have a mystery of some sort in your hands.