Before killing the hero who has been accused of the crime, the villain invariably confesses his entire role to the hero, allowing the hidden police to not only save the hero, but witness the confession simultaneously.

When the detective gathers all the suspects together at the end of the mystery and outlines the crime, the guilty party ALWAYS confesses.

The Mystery Wizard (Marple, Holmes, Poirot, Mike Hammer) is ALWAYS surrounded by bumbling, ineffectual cops who thwart the investigation, and may even accuse the MW of involvement.