With
rational human thought comes a whole slew of
fears created by our
mind's wonderful ability to "fill in the gaps". This not only saves on much
paper and
film, but actually increases the tension as people, when left to their own devices to imagine the
worst, usually
imagine something far worse than the producer had in mind.
Notable examples of this are seen in films like Jaws, Henry:Portrait of a Serial Killer, Se7en, Shawshank Redemption, and others.
The beauty of this approach is that if done properly, the effect is totally believable, while utterly ambigious. In real life it is often the cause of intense and lasting psychological scarring, and is often something that preys on the minds of real life detectives. For example, just today on the news a story about the "cut up" remains of a pensioner found in a suitcase next to the side of a road in the UK. The detective who had to piece this whole saga together probably went through a much worse time than either you or I in reading about this, as he had to recreate the complete picture in his mind from far more information, and with far more possibilities than any of us. This is obviously a double edged sword.
See also: Exams.