The screenplay is the only document that exactly describes the narrative structure of the film. After that, it is broken down to pieces, in order to achieve what it describes. Even the shooting is not done chronologically, and it is a shot-by-shot process. There are many improvisations during shoot that go beyond the screenplay. So by the end of the shooting, the director is left with an enormous amount of footage, millions of shots preserved in cans (and several among them from various camera angles), and with a total loss of objectivity. Now, he needs a person who has been detached with the process to help him put these pieces together. Here comes the editor.
So, Editing is the process of re-constructing the narrative structure of the film, mainly based on the screenplay. But it also incorporates the improvisations, and makes certain decisions regarding the flow of the film, as it is now that the actual film starts appearing. Some scenes may have to go. Some extra shots may need to be added. And apart from ‘film editing’ or splicing the shots together, with the transitions like fades and dissolves, it involves the adding of the soundtrack: voices, ambient sound and background score. In fact, editing is the process that actually makes cinema a unique art-form. Writing, acting, photography, music, art-direction, choreography – all are contributions from other older forms of art. Editing combines them together to make cinema – the art of the moving image.