As a thriller novelist, and as an editor, I often participate in discussions about definitions: what's a thriller? A mystery? A suspense novel? The distinctions simultaenously seem important ("How shall we categorize this book?") and inconsequential ("You either love thrillers or hate 'em"). The thriller genre has become something like the United States: a melting pot of characteristics, with a few distinguishing elements.
Generally speaking: I see thrillers as a supercategory that includes subcategory genres like mystery and suspense. Patrick Anderson's The Triumph of the Thriller does a fantastic job of documenting the rise of thriller novels (specifically crime novels) in popular culture all the way from Edgar Allen Poe and Arthur Conan Doyle to Tom Clancy, Thomas Harris, Karin Slaughter and others. But novelist Janet L. Smith has defined mystery and suspense most succinctly and memorably for me. Drawing upon dramatic explanations offered by Alfred Hitchcock, she suggests that a mystery seeks to answer the question what happened? while a suspense seeks to answer what's going to happen? You'll see if you read her article that I've oversimplified things. But there it is.
The reason Janet's definition caught my attention is because it has helped me consider why I've always been pulled to both mysteries and suspense novels. As an adolescent I loved the intellectual and ethical ponderings of Agatha Christie; later I loved the psychological and sometimes metaphysical adventures of Dean Koontz, which also engage on a moral level. But there is something deeper at work. Thrillers aren't only about the thrill. They're about life: trying to figure out what happened, and dreaming about (or stewing or pondering or predicting or anticipating) what's going to happen. The emotional and spiritual rides are meaningful to me.
So perhaps it shouldn't be too big of a surprise that I find myself writing thrillers now, thrillers with a supernatural and spiritual bent. Because I suspect that the answers to life's questions--what happened? what's going to happen?--don't always have answers rooted in the physical world.