The voiceover is there to serve two main purposes. To inform viewers about the extra-diagetic elements such as the star and the director, and to give an idea as to the film's storyline and genre. The former is factual information, if the audience knows the name of the film's star or director then they are more likely to want to see the film than if they hadn't. The latter is informing the viewer factually as to the plot of the film, but also selling it to them through romantic descriptions and hyperbole. It is here that the advertising factor of trailers comes to the forefront. The trailer itself in terms of the clips and soundbites can be seen as a free sample taken from the actual product whilst the voiceover is a description of the product that they can buy. When audiences choose to watch a particular film at the cinema or rent it on video they are paying money and getting nothing physically back. The film industry is providing an entertainment service and potential audiences need to know that they are not wasting their money. What the voiceover in a trailer serves to do when describing the film poetically is to assure the audience that they are going to get value for their money, that this film is worth going to see. It is a voice coming from outside the diegesis of the film clips telling you to go and see that film. It appears to be giving you a neutral comment when in fact it is far from being neutral.
The gravelly tones of a voiceover artist have come to be one of the most well known features of a trailer. There are two artists that provide the majority of these voiceovers, Dan Lafontaine an American who started off scripting trailers and now works full time recording up to 15 trailers a day, and Red Pepper, an Englishman who started work in the mid nineties and whose trailers now include The Lost World, Con Air, and Armageddon. (S)
Listening to the above sound clips it is hard to notice much of a difference between the two voices. They both have an American accent, the same deep gravelly voice and talk at the same speed. If you did not know that these were two different artists it would be hard to spot. This leads to a similarity between trailers, a unifying bond between genres. A simple change in the tone of voice and it fits to a comedy, drama or action thriller. The only time when a voice other than one of these two artists will be heard in a trailer is when it comes from one of the film's characters (see Jerry Maguire).
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Listen to an example from Red Pepper (S)
Listen to an example from Dan Lafontaine (S)