Conventions of a trailer

Conventions of trailers include a voice over. A voice over is used to help give the audience a grasp of the storyline of the film. In trailers, they usually establish the genre of the film, so that it will attract their target audience. Actors are shown as well because people will be more encouraged to go watch the film if they have an actor that they like to watch in the film.
Trailers also give you details about the film like release date, title, credits etc.

The trailer 'Piranha 3D' shows you that is an thriller/horror film with the music being jumpy and builds tension. The film also shows its unique selling point which is that its in 3D,  as its in the title.  The target audience is established as they show a party at the beach, full of teenagers and young adults. The pace of the trailer is quick with quick editing. There is dialogue throughout the trailor, with quotes from the film being used. Credits are also used, with  a rhetorical question being used 'Can You Swim' as well as giving information about release date, producers and directors.    Watch the Piranha 3D trailer here