What is genre? Genre is a way of classifying movies in a way that makes it easy for the population to choose the type of film they would like to see. Genre allows directors to make a film that a niche audience can relate to. Without genre, directors would find it difficult to create a film that a majority would like to watch, this is because some genre types are larger than others, as people watch certain genres more than others, though the popular genre often depends on the fashion and values of the time, relevant to Voloshinov's theory which states of judging a piece of text based upon the time it was made.
Within genres there are certain cliches that an audience would expect to see. An example of this would be an action movie where the bad guys lose and the good guys win, a romance movie where the couple end up happily married, or a detective movie where the crimes get solved. An even more basic movie cliche is that every thing's normal, something bad happens, it gets fixed, something worse happens, it gets fixed just at the last second. The reason for this is often, the audience expects to see certain cliches in movies and genres, without them the audience is likely to feel uncomfortable or threatened, the only way to abolish this is to follow the set cliches. Although if everyone did this, all movies with a shared would be near copies. The reason they're not is because of experimental directors, like Alfred Hitchcock, he killed off one of the main characters in the middle of his famous thriller film, Psycho (1960).
Genre, in the film industry, is a way that studios and distributors can market their products to standardise or differentiate their output. This means that these companies can familiarise or un-familiarise their products with their consumers. The reason a company may release an unfamiliar product could be to attract a newer audience, this could just be to change the style, make more money from the new audience or to survive as the previous audience may be losing interest.
In the same way a media company can choose to distribute different genres of films, a director can choose to direct different genre, or experiment by combining 2 genres, like rom-com or psychological-thriller. These combine genres are sub-genres. Sub-genres can either be made purposefully or accidentally, because often a film will fit into multiple genres. An example of this would be Fight Club (1999) by David Fincher, the genres that this film covers are as follows: Thriller, Cult, Airplanes & Airports, Action, Action-Adventure, Satire, Drama, Psychological-Thriller, Black Comedy. This massive list of genres that Fight Club fits into was likely to not be intentional as it's doubtful that Fincher set to cover all of those film types. Instead it's likely that the final product fit those genres by accident. Bearing that in-mind, I assume that Fincher set himself the target to fit at least one of those genres, likely psychological-thriller, action or action-adventure. As it is very rare for a director to begin filming without an idea of the genre they would like to cover.
Genres often follow rules and cliches. These rules and cliches can however be broken and changed in an attempt to create something newer or to survive. These creations are sub-genres, they follow a similar, yet unfamiliar pattern to the genres they're made from. Without this pattern of change we would never create a newer idea that others can watch and learn from. Without the idea of genre and genre evolving the movie industry would dead. It is because of genres evolving to produce new ideas that the film industry is where it is today.