Computers always make a noise when something appears on the screen. Warning messages always appear in inch-high letters against a flashing red background while making a repeating raucous noise.

Time bombs always have a digital countdown timer with a large red LED display. Mad bombers always thoughtfully include this feature so that the intended victim/hero will know how much time he has to disarm the bomb.

A computer-controlled bomb (like the one in Aliens) will have a cutoff point in the countdown after which even the person who armed it can no longer disarm it. It will never be explained why this feature was programmed into it, or what law of physics requires it.

In any melodrama (that is, any story with clear-cut good guys and bad guys) there may be a minor character that is part good and part bad. (For example, he may start out good, but will betray the hero in a moment of weakness. Or he may start out evil, but hesitates to carry out his evil boss's ruthless demands.) This character always dies during, or shortly before, the climactic scene. Also, there will be something ironic about his manner of death. (He won't be killed outright but, for example, he might fall into a trap that was meant for the hero.) You might conclude from this that Hollywood hates ambiguity while it LOVES pure evil.

Anyone who dies passes instantly from fully alert consciousness to death, without passing through any intermediate stages like confusion, delirium, unconsciousness, or coma.

Anything a person says on his deathbed will be perfectly intelligible and profoundly meaningful.

The plot development that the director has carefully planned to surprise the audience in the middle of the film will be given away in the trailer. (The trailer is, after all, made by the marketing department of the studio, whose goal is to get you into the theater, not to help you enjoy the film once you're there. Moral: if you want to get maximum enjoyment from a movie, avoid watching trailers.)