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.