Film exhibits an existential time structure. It presents characters and events in the present moment and as the film progresses the future of the characters is built from their past within the film. Characterisation in film differs from that of the novel primarily in that characters are seen as they become what they are through their actions. In the novel a character is already fixed. Film also differs from theatre in that the differing use of space allows for a more intimate acting style on the part of the actor and more involvement on the part of the viewer. Film noir, in particular, manifests existential themes in its plot lines and characterisation, e.g. alienation, anxiety and the developmental nature of personality. A number of theorists have attempted to define film noir and to account for its distinctive look. These include David Bordwell, Borde and Chaumeton and Christine Gledhill. So far no definition has successfully accounted for all the features present in these films. An existential analysis offers a new way to approach this problem. Due to its distinctive use of philosophical themes film noir constitutes a separate and continuing genre. A noir paradigm can be established by isolating these philosophical themes and the stylistic methods which express them. This can be used as a criterion to identify noir films. Noirs can be divided into different periods by identifying the dominant theme of various decades. These reflect the social conditions of the time as well as popular response to existential philosophers, for instance, Sartre and Nietzsche who have both been considerable popularised. In addition noir can be seen as a response to restrictive Hollywood practices.