Many authors anguish over whether they’ve written a mystery, a suspense novel, or a thriller. This is especially important when entering a writing contest, because if the manuscript is submitted in the wrong category it could be disastrous.
A good rule of thumb says mysteries are about whodunit, while suspense/thrillers are about how it was done and how the protagonist is going to avoid having it done to him/her, or to someone she/he loves.
Most murder mysteries begin with a murder, often on a dark and stormy night. Then the novel proceeds with the main character (an amateur or professional sleuth) finding clues that lead to the eventual capture of the murderer.
A thriller/suspense novel is often character driven and depends upon the protagonist gripping the interest of the reader. Often it’s the sleuth’s foibles or personality and character flaws that engage the reader. The main character is then going to match wits with the killer as the chase is on. A thriller/suspense novel often has the main character trying to prevent the villain from committing an even more heinous crime. If your detective, pair of sleuths, or group of protagonists are in a high stakes environment with a great threat looming and the unthinkable about to happen, you’ve probably written a thriller.
But then there are sub-genres. Murder mysteries can be divided into cozies, “locked room” mysteries, food mysteries, pet mysteries, even gardening mysteries. Thrillers can be classified as military or espionage, political, medical, legal, psychological. Just when it looked like it was all figured out, the detective novel messes it all up because some of them are mysteries while others are thrillers. And what about romantic suspense and capers? Yikes!