All storytelling is a combination between learning and acting. Detective stories emphasize the learning step more than all others.

The detective is obsessed with knowing. Indeed, he is a model of how we know anything. In other words, the detective doesn't create reality like the action hero, he recreates it.

That makes the detective story one of the most difficult to structure. For one thing, the detective is always looking backward. He's like an archeologist, digging through the ruins of the crime scene to see if he can tell how these people lived, and how they died. This constant looking backward puts the story in conflict with the need for forward drive, especially in current movies that have to move forward at breakneck speed.

How do you make looking backward move forward? One technique: don't settle for the initial crime to drive the plot. You need an ongoing opposition to keep the conflict going in the present. In other words, you want more murders as the detective tries to solve the case that put the story in motion.