Writing a mystery is a lot like playing with both sides of the same coin. You have to play both sides of the field in a sense in order to have the protagonist come out on top. That is, if that’s what you decide to do. After all, even Sherlock Holmes didn’t solve all of his cases.
Either way, you’ve got to play the field. Who is your unsub? Why did they do what they did? How did they do it? Was it spur of the moment or did they plan it out? What did they leave behind, intentionally or accidentally?
You might be thinking that these are questions your protagonist needs to answer and you would be correct. But in order for the crime to be there, you have to be in ahead of your unsub.
After all, they are the ones who did the crime that you now have to solve. And trust me when I say not knowing everything they’re up to is a pain—when it comes to leading your protagonist to them.
Motive. Books and articles have been written on this subject (hey, look how many of those articles are mine!). And each and every time I start a mystery I spend hours—days—agonizing over why my unsub did it.
Criminal Minds has said motive doesn’t have to make sense to us, it only has to make sense to the killer.
Badge City: Notches is a good example of this, in my opinion. When figuring out my unsub’s motive, I spent a lot of time agonizing over why they were driven to eight little girls. At the end of the day, I realized that it might not make sense to everyone, my readers and protagonist included, but it made sense to them.
But the why can lend itself to some clues that your unsub might purposely or accidentally leave for your protagonist to find.
Leaving clues intentionally might tell you a bit about your unsub. Maybe they want to play a game with the protagonist and their organization. Or they honestly think they are smarter than them. Or that they are being witty. Either way, it’s a way to profile.
There were a lot of ORs or MAYBEs in that last paragraph. Because there are an almost infinite number of possibilities for you to discover and perhaps use.
Knowing who is also helpful. I’ll confess something to you. On the mystery I’m writing right now, I actually didn’t figure it out until I was halfway through and I’m actually still ironing out some of the details.
I do not recommend that route.
There is the well-known rule that you have to introduce your readers to the unsub at some early point. Some people do it in odd ways. They introduce them outright as a suspect or a bystander.
Both routes have their merits and it is up to you to decide which one you want to choose. But seriously, have your readers meet them. Because whether or not they want to admit it, they want to play the guessing game—and doing without introducing your killer reduces him or her to a deus ex machine.
One coin, two sides, and a whole lot of questions. But in the end, life is full of mysteries that can be fun to figure out as they come.