Genre

Why have genre?
Genres have certain conventions which help the audience to identify the type of film it is. If they are able to identify which genre of film it is they will have a rough idea as to whether they will like it or not, and this will either make them interested in watching the film or not, depending on their preferences within genre as a whole.

Whose uses genre?
Genre is used by both the audience and the industry to be able to identify films.

Genre is used within the industry by producers, directors and editors alike to be able to make good decisions about the type of shots that should be used, the type of characters etc. Basically they use the genre to be able to determine the following factors: characters, themes, iconography and narrative.

If conventions within the thriller genre aren't used in a thriller film then it will be difficult to tell it's genre so it won't appeal to the target audience as it is intended to do.

Where do genres come from?
Conventions come from multiple films using ideas from one specific film. By sourcing/referencing these ideas and incorporating them into their own film with a new twist, you get a collection of films with similar ideas. Referencing an idea from a film and using it in another is called intertextuality.

Overtime these group of films evolve into genres, this is because the audience like that type of film so they go to see them in the cinema, as a result film companies make more films of this genre so that they can make a profit.

Genres evolve to suit changing audiences. Remakes of films, such as Disturbia being a remake of Rear Window, use the core elements of the original but edit them to suit a different audience.

Sub-genres.
Sub-genres occur when lots of films of a certain genre have been made. This is because when there are a large amounts of a genre you get certain groupings within that genre, this then leads to sub-genres.

Sub-genres are a specialised version of a specific genre, however it has to be distinctive within that genre that there is a relatively large group with the same characteristics.