What’s the difference between a mystery and a thriller? Admittedly, more and more these days, genre-straddling and genre-mixing books are appearing, but traditionally, there is a difference between the mystery and the thriller.
Mysteries are all about the crime. Usually a murder–because that is the ultimate crime in this kind of book–has been committed or is about to be committed. Mysteries are books where the emphasis is on detection and solving the mystery, often with clues for the reader to solve along too. The important thing for the mystery is answering the question ‘whodunit’? Mysteries are also concerned with character, motive, and with the details of crime solving. If the mystery is a police procedural, there will be more about the ‘official’ approach to crime and the detective must operate within the constraints of the law, but is aided by forensic science and can question suspects more openly, whereas in the traditional amateur sleuth story, there will be a more relaxed approach and the sleuth is free from legal restraints to conduct the investigation in a different, sometimes novel, way. For example, the methods of Sherlock Holmes or Hercule Poirot. Often the amateur sleuth must detect in secret.
By contrast, thrillers are a race against time. The main thrust of a thriller is the pressure to solve a crime or prevent a crime or major disaster before ‘it’s too late’. Thrillers often cover a lot of ground – the protagonist may go from country to country or city to city in search of the answers, where mysteries are usually centred around a single location. Thrillers are less about detection and clues, and much more about tension, pace, and events. Usually minor characters are fewer and less important to the investigation, and crime or murder is not always the main thrust of the story. There may or may not be a detective, but where used, these are usually professionals. If there is murder, it is high profile, often a serial killer. For example, experts Alex Cross or Robert Langdon.