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A film genre is a motion-picture category based (for example) on similarities either in the narrative elements or in the emotional response to the film (namely: serious, comic, etc.). Most theories of film genre are borrowed from literary-genre criticism. Each film genre is associated[by whom?] with "conventions, iconography, settings, narratives, characters and actors".  Standard genre characters vary according to the film genre; for film noir, standard characters are the femme fatale and the "hardboiled" detective; a Western film may portray the schoolmarm and the gunfighter. Some actors acquire a reputation linked to a single genre, such as John Wayne (the Western) or Fred Astaire (the musical). A film's genre will influence the use of filmmaking styles and techniques, such as the use of flashbacks and low-key lighting in film noir, tight framing in horror films, fonts that look like rough-hewn logs for the titles of Western films, or the "scrawled" title-font and credits of Se7en (1995), a film about a serial killer. As well, genres have associated film-scoring conventions, such as lush string orchestras for romantic melodramas or electronic music for science-fiction films.
The basic genres include fiction and documentary, from which subgenres have emerged, such as docufiction and docudrama. Other examples of subgenres include the courtroom- and trial-focused drama known as the legal drama, which is a subtype of drama. Types of fiction which may seem unrelated can also be combined to form hybrid subgenres, such as the melding of horror and comedy in the Evil Dead films. Other popular combinations include the romantic comedy and the action comedy film. Alan Williams distinguishes three main genre categories: narrative, avant-garde and documentary. Genre movies are "commercial feature films which, through repetition and variation, tell familiar stories with familiar characters and familiar situations". Genre affects how films are broadcast on television, advertised, and organized in video rental stores.
Films can also be classified by the setting, theme, topic, mood, format, target audience or budget. The setting is the environment where the story and action take place (e.g., a war film, a Western film, or a space-opera film). The theme or topic refers to the issues or concepts that the film revolves around (e.g., science-fiction film, sports film, or crime film). The mood is the emotional tone of the film (e.g., comedy film, horror film, or tearjerker film). Format refers to the way the film was shot (e.g., 35 mm, 16 mm or 8 mm) or the manner of presentation (e.g.: anamorphic widescreen). Additional ways of categorizing film genres may involve the target audience (e.g., children's film, teen film or women's film) or by type of production (e.g., B movie, big-budget blockbuster or low-budget film). Genre does not just refer to the type of film or its category; spectator expectations about a film, and institutional discourses that create generic structures also play a key role. Genres are not fixed; they change and evolve over time, and some genres may largely disappear (for example, the melodrama).
The term "genre" was used to organize films according to type since the earliest days of cinema. By the 1950s, André Bazin was discussing the concept of "genre" by using the Western film as an example; during this era, there was a debate over auteur theory versus genre. In the late 1960s, the concept of genre became a significant part of film theory.
Film genres draw on genres from other forms; Western novels existed before the Western film, and musical theatre existed before film musicals were made. The perceived genre of a film can change over time; for example, The Great Train Robbery (1903) is seen in the 2010s as a key early Western film, but when it was released, it was seen as related to the "then-popular genres of the chase film, the railroad film and the crime film". A key reason that the early Hollywood industrial system from the 1920s to the 1950s favoured genre films is that in "Hollywood's industrial mode of production, genre movies are dependable products" to market to audiences, they are easy to produce and it is easy for audiences to understand a genre film. In the 1920s to 1950s, genre films had clear conventions and iconography, such as the heavy coats worn by gangsters in films like Little Caesar (1931). The conventions in genre films enable filmmakers to create them in an industrial, assembly line fashion, an approach which can be seen in the James Bond spy films, which all use a formula of "lots of action, fancy gadgets, beautiful woman and colourful villains", even though the actors, directors and screenwriters changed. 
Films are rarely purely from one genre, which is in keeping with the cinema's diverse and derivative origins, it being a blend of "vaudeville, music-hall, theatre, photography" and novels. American film historian Janet Staiger states that the genre of a film can be defined in four ways. The "idealist method" judges films by predetermined standards. The "empirical method" identifies the genre of a film by comparing it to a list of films already deemed to fall within a certain genre. The apriori method uses common generic elements which are identified in advance. The "social conventions" method of identifying the genre of a film is based on the accepted cultural consensus within society. Martin Loop contends that Hollywood films are not pure genres because most Hollywood movies blend the love-oriented plot of the romance genre with other genres. Jim Colins claims that since the 1980s, Hollywood films have been influenced by the trend towards "ironic hybridization", in which directors combine elements from different genres, as with the Western/science fiction mix in Back to the Future Part III.
Many films cross into multiple genres. Susan Hayward states that spy films often cross genre boundaries with thriller films.  Some genre films take genre elements from one genre and place them into the conventions of a second genre, such as with The Band Wagon (1953), which adds film noir and detective film elements into "The Girl Hunt" ballet. In the 1970s New Hollywood era, there was so much parodying of genres that it can be hard to assign genres to some films from this era, such as Mel Brooks' comedy-Western Blazing Saddles (1974) or the private eye parody The Long Goodbye (1973).  Other films from this era bend genres so much that it is challenging to put them in a genre category, such as Roman Polanski's Chinatown (1974) and William Friedkin's The French Connection (1971).
Film theorist Robert Stam challenged whether genres really exist, or whether they are merely made up by critics. Stam has questioned whether "genres [are] really 'out there' in the world or are they really the construction of analysts?". As well, he has asked whether there is a "... finite taxonomy of genres or are they in principle infinite?" and whether genres are "...timeless essences ephemeral, time-bound entities? Are genres culture-bound or trans-cultural?" Stam has also asked whether genre analysis should aim at being descriptive or prescriptive. While some genres are based on story content (the war film), other are borrowed from literature (comedy, melodrama) or from other media (the musical). Some are performer-based (Fred Astaire and Ginger Rogers films) or budget-based (blockbusters, low budget film), while others are based on artistic status (the art film), racial identity (Race films), location (the Western), or sexual orientation (Queer cinema).
Many genres have built-in audiences and corresponding publications that support them, such as magazines and websites. For example, horror films have a well-established fanbase that reads horror magazines such as Fangoria. Films that are difficult to categorize into a genre are often less successful. As such, film genres are also useful in the areas of marketing, film criticism and the analysis of consumption. Hollywood story consultant John Truby states that "...you have to know how to transcend the forms [genres] so you can give the audience a sense of originality and surprise."
Some screenwriters use genre as a means of determining what kind of plot or content to put into a screenplay. They may study films of specific genres to find examples. This is a way that some screenwriters are able to copy elements of successful movies and pass them off in a new screenplay. It is likely that such screenplays fall short in originality. As Truby says, "Writers know enough to write a genre script but they haven't twisted the story beats of that genre in such a way that it gives an original face to it".
Cinema technologies are associated with genres. Huge widescreens helped Western films to create an expansive setting of the open plains and desert. Science fiction and fantasy films are associated with special effects, notably computer generated imagery (e.g., the Harry Potter films).
There are other methods of dividing films into groups besides genre. For example, auteur critics group films according to their auteur-directors. Production attributes, such as the low-budget film, can also be considered a grouping. Some groupings may be casually described as genres although the definition is questionable. For example, while independent films are sometimes discussed as if they are a genre in-and-of themselves, independent productions can belong to any genre. Similarly, while art films are referred to as a genre by film scholar David Bordwell, who states that "art cinema itself is a [film] genre, with its own distinct conventions", an art film can be in a number of genres (e.g., drama, experimental film, black comedy, etc.).
Because genres are easier to recognize than to define, academics agree they cannot be identified in a rigid way. Furthermore, different countries and cultures define genres in different ways. A typical example are war movies. In US, they are mostly related to the two World Wars whereas, in other countries, movies related to wars in other historical periods are considered war movies.
Film genres may appear to be readily categorizable from the setting of the film. Nevertheless, films with the same settings can be very different, due to the use of different themes or moods. For example, while both The Battle of Midway and All Quiet on the Western Front are set in a wartime context and might be classified as belonging to the war film genre, the first examines the themes of honor, sacrifice, and valour, and the second is an anti-war film which emphasizes the pain and horror of war. While there is an argument that film noir movies could be deemed to be set in an urban setting, in cheap hotels and underworld bars, many classic noirs take place mainly in small towns, suburbia, rural areas, or on the open road.
Linda Williams argues that horror, melodrama, and pornography all fall into the category of "body genres" since they are each designed to elicit physical reactions on the part of viewers. Horror is designed to elicit spine-chilling, white-knuckled, eye-bulging terror; melodramas are designed to make viewers cry after seeing the misfortunes of the onscreen characters; and pornography is designed to elicit sexual arousal. This approach can be extended: comedies make people laugh, tear-jerkers make people cry, feel-good films lift people's spirits and inspiration films provide hope for viewers.
A genre movie is a film that follows some or all of the conventions of a particular genre, whether or not it was intentional when the movie was produced.
In order to understand the creation and context of each film genre, we must look at its popularity in the context of its place in history. For example, the 1970s Blaxploitation films have been called an attempt to "undermine the rise of Afro-American's Black consciousness movement" of that era. In William Park's analysis of film noir, he states that we must view and interpret film for its message with the context of history within our minds; he states that this is how film can truly be understood by its audience. Film genres such as film noir and Western film reflect values of the time period. While film noir combines German expressionist filming strategies with post World War II ideals; Western films focused on the ideal of the early 20th century. Films such as the musical were created as a form of entertainment during the Great Depression allowing its viewers an escape during tough times. So when watching and analyzing film genres we must remember to remember its true intentions aside from its entertainment value.
Over time, a genre can change through stages: the classic genre era; the parody of the classics; the period where filmmakers deny that their films are part of a certain genre; and finally a critique of the entire genre. This pattern can be seen with the Western film. In the earliest, classic Westerns, there was a clear hero who protected society from lawless villains who lived in the wilderness and came into civilization to commit crimes. However, in revisionist Westerns of the 1970s, the protagonist becomes an anti-hero who lives in the wilderness to get away from a civilization that is depicted as corrupt, with the villains now integrated into society. Another example of a genre changing over time is the popularity of the neo-noir films in the early 2000s (Mulholland Drive (2001), The Man Who Wasn't There (2001) and Far From Heaven (2002); are these film noir parodies, a repetition of noir genre tropes, or a re-examination of the noir genre?
This is also important to remember when looking at films in the future. As viewers watch a film they are conscious of societal influence with the film itself. In order to understand it's true intentions, we must identify its intended audience and what narrative of our current society, as well as it comments to the past in relation with today's society. This enables viewers to understand the evolution of film genres as time and history morphs or views and ideals of the entertainment industry.
AFI defines 'western' as a genre of films set in the American West that embodies the spirit, the struggle and the demise of the new frontier.
[...] the various elements of genre films, including conventions, iconography, settings, narratives, characters and actors.
Alan Williams (quoted in Neale, 1990: 62) speaks of 'principal genres' to refer to what he sees as the three main categories of film: narrative film, avant-garde film and documentary. He reserves the term sub-genres to refer to what we term film genres.