Sherlock Holmes, Hercule Poirot and Jane Marple invariably know what they're doing, but the comically inept Inspector Clouseau-like sleuth is one of the mystery movie's tried-and-tested conventions.

A look at how comic relief helps to offset the grisliness of murder, but also how the detective's seeming ineptitude – sometimes a deceptive act – encourages the viewer to play along and figure out the solution first.