From the NPR web site, a good quote for discussion:
Which raises the question, what defines a thriller? Clearly it's not setting or subject matter.
For the purposes of this contest, we'll stick with the answer James Patterson once gave, which is that thrillers are defined by the "intensity of emotions they create ... of apprehension and exhilaration, of excitement and breathlessness. ... By definition, if a thriller doesn't thrill, it's not doing its job." If the closely related mystery genre is about discovery, then thrillers are more oriented towards action and suspense. The villain may be known from the start; the fun comes from finding out how the hero will foil whatever evil plans are afoot.