A trailer is a film's contract with the audience, telling the audience what to expect from the actual film. The audience can expect the film to conform to the genre indicated in the trailer. They can expect to see the film's hook in the trailer. They can expect to see snippets of the film's story in the trailer, and they can expect these snippets not to mislead them. And for each cool moment in the trailer, they can expect several even cooler moments in the film itself.

If this contract is broken, the audience will leave the film dissatisfied. Suppose the trailer indicates the wrong genre or takes a minor plot point and disguises it as the hook or includes scenes that aren't really important in the film itself or, worst of all, includes every single cool moment in the film. This is tempting if the film isn't very good, because the trailer can make it look better than it is. But it's never a good idea, because a breach-of-contract trailer, while it may have been riveting, was promising something it didn't deliver. The quality of the actual film doesn't matter if people who chose to see the film, based their decision on the trailer -- and didn't get what they were promised.

It's vital to stick to the contract. And in addition to including the actual genre, hook, some plot points, and a few cool moments, it's also important to avoid including certain things. For instance, if the film has a major twist at the end, that should absolutely not go in the trailer. For a comedy, the film's first big laugh should not go in the trailer (the theatrical trailer for Death at a Funeral (2007) broke this rule, and as a result the first laugh fell flat for anyone who had seen the trailer). The plot points in the trailer should be enticing but should rarely reveal much beyond the first act, and in no way should a trailer give away the film's resolution. And the film's very coolest moments should be saved for the film itself.

The trailer is just a taste. The film is the meal.

A good trailer will bring the right people to see the film, people who will like it. A trailer that breaks its contract with the audience will bring people who will be disappointed by the film and will spread the word about how bad they thought it was. And no film deserves to be brought down by its own bad trailer.