When marketing and studying cinema it is important to know whom the intended audience are. For example when studying the classics such as Gilda we must take into account at what time the film was made and for which audience. We know that it was created after the Second World War and that the role of the woman was changing and this was an important element to the audience of the time because it was re-defining the fabric of the society.
An important part of Genre theory is knowing at which period a film was made. This is particularly important when considering what technology was being used. With today's high speed films (shorter exposure times) films may be shot with very different lighting from those of twenty years ago. The rhythm and pacing of the film may have been altered with editing. Core values which would seem unimportant to us when watching certain films may be of prime importance to the people whom are contemporary to the film's.
Genre theory is a very good way of marketing a film both for the producers, secondly for the audience. It has become a logo of sorts marking which films certain people may want to see. In particular we may notice this with the teen movie genre. It is a genre which is divided into many sub genres such as spoof etc.
If all films were exactly the same then genre theory would be easy to study and defining which type of film it is would be easy. Unfortunately though it is far more complicated than this. As we look and analyze various films we find that although they have similarities in other aspects they are very different. It creates the need for genre theory to continually re-define itself according to the technology, the audience, the political message among many other variables. What may have been typical of a genre at one time may evolve other time as the audience evolve in such a way that an action film from the a few years ago may be a very slow paced film by today's standards, compare for example the Western with the action film.