It is very important for a trailer not to be misleading in terms of genre more than any other factor. Audiences tend to have genres that they like and don't like and misleading them in this area will greatly affect their enjoyment of the film, probably for the worse. To market an Action film as a Romance for example would attract an audience with expectations for the film that could never be met. They would then generate extremely bad word-of-mouth, which is what a film relies on after the opening weekend. How then does a trailer seek to effectively show us the genre of the film that it is marketing? The answer is fairly simple;
"Viewers prefer to know what kind of a story they're about to watch unfold, the range of pleasures and the kinds of surprises they can expect. As a result, genre comes second only to the star as a marketing hook." (S)
Like the star aspect, definition of genre in a film's trailer has a lot to do with audience expectation. If a trailer can effectively establish the type of film that it is promoting it can attract the right kind of audience for that genre.
"if you have a horror movie that you're advertising, you make a scary trailer, something that will make people in the theatre jump. If you have a comedy, you make the funniest trailer that you can." (S)
Watch this trailer for the 1998 Eddie Murphy comedy Dr Doolittle and notice how this strategy is used. All of the soundbites used are jokes, thus making a funny trailer for a funny film. There should be no doubt in any audience's mind that this film is a comedy. The music used is also an obvious indicator of genre as is discussed elsewhere in this project.