A lot of best-sellers come out of these genres, because if you do it right, the whole idea is to keep people breathlessly turning the pages until long past bedtime. The difference between whether something is marketed as a thriller or as a suspense novel may depend on which is selling better this year: they’re very close, the way fantasy and science fiction shade over at science fantasy.
We could define the thriller as the novel where things keep happening to keep the protagonist jumping and suspense as the novel where things keep hanging over the protagonist. I have a tendency to call suspense novels “serial killer novels” because so many I see on the supermarket racks feature just that, with a former near-victim needing to counter-hunt the killer in order to escape again.
Various editor’s blogs and writer’s advice sites over the last ten years have told me that the venerable realm of horror is perhaps better called supernatural thriller or supernatural suspense in your cover letter. This fashion may shift any time and, since I only trip over these by accident (not being a horror writer myself), be sure and do your research. Certainly, “horror” is a lot shorter to keep typing on your writers’ forums.
What the other highlights is that a horror story or novel needs more than horror: it needs tension and pacing ratcheted way up rather than being a leisurely display of horrible things. It works on the thriller or suspense templates, only the menace or villain is a supernatural one, or at least way off the known edge of science. I keep thinking of Whitley Streiber’s The Wolfen, in which the supernatural menace of werewolves turns out merely to be a race of hyper-intelligent wolves, which was labeled horror but couldn’t really have been called supernatural anything. Myself, I would label it dark science fiction, but horror was outselling sf by a mile back then.
This comes down to my repeated line that genres and genre names and boundaries are marketing tools. The publishers have found modern horror readers want the intense pull of the suspense or thriller genres, along with gruesome terrors. A leisurely conte cruel just doesn’t sell big any more.
A special sub-genre of thrillers (not usually suspense) is espionage fiction or spy fiction (all those Len Deighton Smiley novels, and I think the Bourne novels can be considered classics) where the protagonist is in the spy game.
Of course, suspense and thriller being kinds of plots, they hybrid instantly with genres that are defined by settings. You wouldn’t have a suspense mystery — you have to choose one sort of plot or the other — but you can have historical thrillers and historical suspense stories, or near-future thrillers, where scientific knowledge and gadgetry have advanced but society not changed beyond recognition.
The latter overlaps with the techno thriller, where the fate of the world hangs on the use of high technology. The classic here is The Hunt for Red October. It’s now set in our past, though at the time it was published it was supposed such a sub might be produced with present technology, and certainly the US subs were simply what were really out there (and possibly a bit less). These always remind me of hard-science science fiction, in that readers are expected to like and enjoy and look forward to descriptions and explanations of the gear that, in any other genre, would be considered horse-choking info-dumps.
This is one reason why, for revision, you need to choose your genre to choose your audience. The people who buy techno thrillers want those big explanations you have, if you make them interesting rather than plonking. The same story, aimed at romantic suspense, even regular thriller, is going to have black boxes with blinking lights that do things, but no one wants to stop to hear the physics of how it does it. This completely changes what you do in revising these areas.
A final sub-genre is the political thriller — think The Manchurian Candidate or Parallax View or The Whisper of the Axe. Someone is jockeying for control of the world, or a country, or at least a city, and as the story goes on the protagonist realizes they will do anything to get that power. But they’re evil and must be stopped. Well, okay, sometimes these have unhappy endings where the villain seizes power anyway, but they’re rare. Politics seems to be one area where readers seem to be okay with being with the hero for 100,000 words only to mostly lose, probably because we all experience parts of government we hate, from the late election’s result to those expensive ugly-lenses they use at the DMV to take your ID pictures.