“Genuine clues” point to the killer and help the sleuth solve the crime. They’re often subtle. The sleuth noticed that the gym teacher was wearing a brown necktie before the Principal was found dead, and now he’s wearing a blue one. A good rule of thumb is to plant three genuine clues in your murder mystery to give your sleuth (and your reader) a good chance of figuring it out. And, for your sleuth to have an airtight case against the killer, one clue should show motive, one proves means and the third proves opportunity.
“Fake clues” point to viable suspects, but who ultimately prove innocent of the murder. You’ll often hear these called “red herrings” and they serve to distract the reader (and sometimes the detective). These clues are often very loud and blatant. The History Professor announces to everyone in the gymnasium that he will get even with the Principal, if it’s the last thing he ever does. Then an hour later, the Principal turns up dead. You can have several red herrings in your mystery, but make sure they lead somewhere. Red Herrings can easily turn into subplots. If they aren’t resolved, they’ll feel like loose ends.
The “Pivotal clue” is the key element that directs the story to the solution—it’s the final piece of the puzzle and, ultimately, allows the sleuth (and the reader) to solve the crime. This clue generally shows up at the end of the middle segment, and leads the characters to a Big Show Down with the Big Bad. This clue can be many things – a lie is uncovered, a truth is revealed, another character returns, evidence at the crime scene that suddenly make sense – and it’s generally dramatic. One of my favorite Pivotal Clues is about what’s missing from the crime scene and should be there. Those are often tough to detect, but seem very obvious when revealed. In a lot of mysteries, the significance of the “Pivotal Clue” is understood but not explained by the sleuth. Ultimately though, it leads him or her to confront the murderer, where the identity is finally revealed.
All three types of clues are essential to a good mystery. It’s the reader’s job to spot them and decode them; it’s the author’s job to inject them into the story in a way that keeps the reader guessing.