The crime fiction genre is a wide umbrella, stretching from murder mystery via police procedurals, all the way to thrillers, each one requiring different tools. In a murder mystery , everything is about the plot. Who did it, how, and why, are the questions that both the protagonist and the readers try to solve simultaneously, normally without any expectations of realism.
A police procedural , by comparison, is all about realism, while battling the challenges of the justice system in order to catch the murderer.
In a thriller , the suspense is built around a challenge posed to the protagonist — the last big score before retiring, being innocently accused of a crime or accidentally getting involved in something dangerous and deadly. And finally, in a psychological thriller, nobody