While I plot and begin writing my second, I thought I'd revisit the basics of mystery writing.
First, the characters:
Protagonist--this is the detective, professional or amateur, who will solve the crime.
Villain--this is the person (or people) who committed the crime.
Victim--the person (or people) who is harmed (often killed) by the crime.
Now, you could have a mystery with only these people, but it would be a little too easy to solve. So lets add some more.
Red herrings--these are other people who might have had motive, means, or opportunity to kill the victim. This complicates the mystery.
A sidekick--a helper for the detective. A sidekick is also a help to the writer, since the detective and sidekick can discuss the case, revealing all kinds of information to the reader in dialogue.
Antagonist--this person may not have committed a crime, but makes life difficult for the protagonist. Which, in fiction, is a good thing.
Experts--especially if the detective is an amateur, he or she may need to draw on the input of experts. Cops, coroners, private detectives--these may figure in the story, often in conflict with the amateur detective.
Family, friends, and romantic interest--no man is an island. But the more family a detective has, the more people the author has to account for. But they do make for interesting subplots and interactions.
Community--general people the characters interact with. They may provide a number of service from yielding clues, making lunch, or just rounding out the picture.
And thanks, Shelly, for pointing out the typo.