Less sophisticated ‘slashers” depend mainly on blood and guts for their shock value. Better-made thrillers — like Psycho or Jaws — precede the violent episodes with psychological twists and foreshadowing.
In Jaws, for example. the shark isn’t seen until an hour and a half into the movie. Instead of graphic violence, the tension is heightened by narrative. Using only shots of the actor’s face, the camera focuses on the horror he remembers as the survivor of a shark attack during his World War II service. When the shark actually appears, its devastation is much more unnerving.