Thrillers is a very broad genre and it has many overlapping sub-genres. The main point of a thriller is to obviously thrill the viewer. They do this through a fast paced plot and with frequent action and a lot of suspense. Thrillers usually centre around only one main character, but they can be with a group too. The main difference between thrillers and horrors is that by the end of the film in thrillers the equilibrium is shifted back to normal but in horrors the equilibrium is still shifty. Thriller usually encompasses a lot of different types of genres, most notably horror and romance. In thrillers, the life of the protagonist or characters he cares about is usually threatened at some point.
The opening of a thriller usually has some sort of action in it or it explains the back-story. An example for an action sequence at the beginning is The Dark Knight, and for the back-story is the start of I Am Legend. Both of these films also have elements of the action genre in them as well.
