Crime fiction typically revolves around a crime, often a murder, which seems impossible to solve. This seemingly unsolvable crime sets things in motion and throws the story down a path to somehow find out what happened. Sometimes the crime takes place on the page, and other times the characters only discover the aftermath. The crime is the heart of the story, and its seemingly unsolvable quality hooks the reader and the protagonist. It becomes the driving motor of the book.
An Interested Sleuth
Even if the protagonist starts out as apathetic to the crime, he must at some point become interested, if not obsessed, with solving the mystery. When Sir Arthur Conan Doyle introduced Sherlock Holmes to the world, an interested detective became a frequently used protagonist in many crime fiction novels. The investigation typically takes the sleuth into terrifying situations, where often the sleuth's life gets threatened, but he cannot give up. He must fight to not only stay alive, but also to solve the crime.
While the degree of depravity varies in crime fiction, some unsavory character typically commits the act. Unsavory characters also typically populate crime stories in minor roles. These characters might be thugs, gangsters or petty thieves that the sleuth must investigate, question and possibly trust in order to solve the mystery.
Danger and Tension
Crime fiction typically continues to increase the tension until the final conclusion. The closer the sleuth gets to solving the crime, the more danger she faces, either from other characters or the environment. While crime fiction can take place anywhere in the world, the setting often helps create tension and danger, such as when Sherlock Holmes ventures into London's seedy underbelly or Mikael Blomkvist must survive the harsh winter conditions in Stieg Larsson's "The Girl with the Dragon Tattoo."