Trailers or previews are film advertisements for films that will be exhibited in the future at a cinema, on whose screen they are shown. The term "trailer" comes from their having originally been shown at the end of a film programme. That practice did not last long, because patrons tended to leave the theater after the films ended, but the name has stuck. Trailers are now shown before the film (or the A movie in a double feature program) begins.
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 Janet Leigh 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. This is nearly always a requirement, as trailers and teasers are created long before the composer has even been hired for the film score...sometimes as much as a year ahead of the movie's release date -- while composers are usually the last creative people to work on the film.
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.
Studios may create trailers in-house or may "farm out" creation to one or more advertising agencies. Agencies that specialize in creating trailers are known as trailer houses. Depending on the amount of influence the filmmakers have with the studio, they may or may not be involved in the creation of the trailer for their film. Some choose to closely supervise the process, when possible. They usually don't get involved until a version of the trailer has been approved by the studio internally.
The producers and editors of a trailer will be given material from the studio to work with, which may include the movie itself (if it has been edited together yet), rushes, and/or computer graphics shots (as they are created during the film editing process).
The trailers that are seen in theaters have been through an extensive process of revisions and approvals by a variety of studio marketing executives. The revision process often includes information from market research conducted at locations all around the country.
Movie trailer editing benefits greatly from the use of a non-linear editing system. Most of prominent trailer cutting companies, Aspect Ratio, Trailer Park, and New Wave Media, have their editors work on Apple's Final Cut Pro.
Beside creative and unique trailer creation, there are several experimental approaches to generate movie trailers automatically using artificial intelligence. University projects like SVP (University of Bremen, Germany) analyse the movie and try to create--- a trailer based on typical structures and rules of trailers.
Trailers that want to have an MPAA rating must submit the trailer for their comments and official rating. This process may take days to weeks. In addition, the trailer must go through TASA certification to regulate sound volumes across theaters.
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