“Genres are dependent upon audiences for both their existence and meaning.” 
It can be said that genre films are ‘manufactured’ or designed to fit a purpose. Much like how a film can be made within the ideas of the auteurism of a particular director, with which a spectator can relate to, so too can it be done with genre. A viewer may have a favourite genre of film and will have an expectation or demand of what they want a film to do – to make them laugh, cry or be frightened, for a few simple examples. A film will be judged, therefore, against the viewer’s pre-existing expectation based on their knowledge or idea of the genre of which it is defined by or marketed as. A film labelled as a horror which doesn’t make the audience frightened is likely to be received poorly, and disappoint horror enthusiasts, despite what other qualities the film may have. Evidently, this has been used by film industry marketers to sell as many cinema tickets as possible by making sure that the ‘right’ audience is coming to see the film. Most cinema-goers prefer familiarity when deciding to see a film, and do not want to be disappointed.
That being said, unexpected genre ‘shifts’ can give a film extra dimensions and meanings. As Grant explains, Peckinpah’s Bring Me The Head of Alfredo Garcia (1974) is initially set up as a western, complete with horses, guns et al., before cars, motorbikes, jet planes and other technologies are shown, revealing the film’s contemporary setting. This has the effect of applying certain elements of the western genre to a film which wouldn’t be considered a western under normal circumstances. Themes of redemption and ‘gunning down outlaws’ make us view and think of this more modern action film as if it were a western.
In addition to this, many films, particularly modern ones, do not fit into only one genre, but often utilise ideas and iconography found in several genres. Sci-fi films Blade Runner (1982) and Dark City (1998) both have elements of film noir (the complicated, troubled anti-hero), westerns (the idea of the lone hero’s struggle) and romance. Bring Me The Head of Alfredo Garcia and Joel and Ethan Coel’s No Country For Old Men (2007) may be considered ‘Neo-Westerns’ as they combine elements of classical western films into a modern action film.
These all, however requires the viewer to have a previous knowledge and familiarity of the relevant genres, to understand the implications being made.
It can be difficult, however, to determine what actually constitutes a particular genre and how we should categorise them. More specifically, what constitutes the ‘limits’ of a genre, for example, when does a thriller become a horror? As Andrew Tudor  notes:
“…we must first isolate the body of films which are ‘Westerns’. But they can only be isolated on the basis of the ‘principal characteristics’ which can only be discovered from the films themselves after they have been isolated.”
He offers the solution:
“Genre is what we collectively believe it to be.”
It is virtually impossible to categorise a genre film without reference to other similar films. A pure, non-subjective starting point is non-existent, and even comparison to a ‘classical’ genre film is unreliable due to evolution of films and genres throughout the years. It is more useful to identify recurring tendencies and social ideologies towards genres and use these as reference.
Genre critic Lawrence Alloway suggests the term ‘iconography’ for familiar, recurring elements found in genre films which have deeper meanings, culturally and historically, beyond those simply in context to a particular film. These may be anything from a stereotypical character to a familiar mise-en-scene or cinematography style. An example of this is the ‘creepy child’ which has appeared in countless horror films such as Village of the Damned (1960), The Omen (1976), The Shining (1980) and The Ring (2002). These are particularly effective, not just in the context of the film, but because of the universal agreement that creepy children are generally considered pretty terrifying and unsettling (Partly thanks to these films themselves!). The presence of one in a film will most likely instantly define it as part of the horror genre.
Certain iconographies may be suspect to cliche, however, and even parody. Mel Brooks’ Blazing Saddles (1974) or Wes Craven’s Scream (1996) are both ‘self-aware’ of various iconography found in genre films and reference these for comedic effect. This also requires the audience’s knowledge and familiarity of genre and iconography.
 Extract from Barry Keith Grant, Film Genre: From Iconography to Ideology (London: Wallflower, 2006).
 Andrew Tudor, Theories of Film (London: Secker and Warburg, 1973), p.139.