Drama is life with the dull bits cut out.
– Alfred Hitchcock
On its most fundamental level, film editing is the art, technique, and practice of assembling shots into a coherent sequence. The job of an editor isn’t simply to mechanically put pieces of a film together, cut off film slates, or edit dialogue scenes. (From Wikipedia)
Despite what actors, directors or even cinematographers may say, the story is crafted in the editing bay or now mostly on a NLE (Non-linear editing) program. A director may have had 25 takes of one shot, but the editor decides which one to use, how long it will be shown and what comes before and after that shot. While a cinematographer and director compose for a each shot, the editor is the composer of the entire symphony of shots/scenes. The editor decides on the speed, direction, which “instruments” to use, how loud or how soft, from crescendos to pianissimo all to tell the story. An editor also speaks the language of cinema but puts the shots of the story into an unique and understandable progression.
The directing, acting, cinematography, musical score, visual effects all are there to serve the editing process, editing is the sum of all these parts.
Walter Murch editor of Apocalypse Now and The English Patient
Walter Murch on editing