From The Art and Popular Culture Encyclopedia
Trailers normally consist of a series of selected shots from the film being advertised. Since the purpose of the trailer is to attract an audience to the film, these excerpts are usually drawn from the most exciting, funny, or otherwise noteworthy parts of the film but in abbreviated form and without producing spoilers. For this purpose the scenes are not necessarily in the order in which they appear in the film. A trailer has to achieve that in less than two and a half minutes, the maximum length allowed by theaters. Each studio or distributor is allowed to exceed this time limit once a year, if they feel it is necessary for a particular film.
Some trailers use "special shoot" footage, which is material that has been created specifically for advertising purposes and does not appear in the actual film. The most notable film to use this technique was Terminator 2: Judgment Day, whose trailer featured elaborate special effects scenes that were never intended to be in the film itself. One of the most famous "special shoot" trailers is that used for the 1960s thriller Psycho which featured director Alfred Hitchcock giving viewers a guided tour of the Bates Motel, eventually arriving at the infamous shower. At this point, the soft-spoken Hitchcock suddenly throws the shower curtain back to reveal Vera Miles with a blood-curdling scream.
The people who create trailers often begin their work while the movie is still being shot. Since the edited movie does not exist at this point, the trailer editors work from rushes or dailies. The trailer may be created at the agency while the movie itself is being cut together at the studio. Thus, the trailer may contain footage that is not in the final movie, or the trailer editor and the movie editor may use different takes of a particular shot. Another common technique is including music on the trailer which does not appear on the movie's soundtrack.
Some trailers that incorporate material not in the movie are particularly coveted by collectors, especially trailers for classic films. For example, in a trailer for Casablanca the character Rick Blaine says "OK, you asked for it!" before shooting Major Strasser, an event that does not occur in the final film.