Roger Ebert on trailers:
"Trailers use exactly the same principle as supermarket demonstrations that supply a sample of cheese on a toothpick. Once you eat it, you know exactly how the cheese will taste. All you lack is having eaten the whole cheese." Source
Which reminds me of this Robert Zemeckis quote:
"We know from studying the marketing of movies, people really want to know exactly every thing that they are going to see before they go see the movie. It's just one of those things. To me, being a movie lover and film student and a film scholar and a director, I don't. What I relate it to is McDonald's. The reason McDonald's is a tremendous success is that you don't have any surprises. You know exactly what it is going to taste like. Everybody knows the menu."