the role of an editor (From Wiki)

Film editing is part of the creative post-production process of filmmaking. It involves the selection and combining of shots into sequences, and ultimately creating a finished motion picture. It is an art of storytelling. Film editing is the only art that is unique to cinema, separating film-making from other art forms that preceded it (such as photography, theater, dance, writing, and directing), although there are close parallels to the editing process in other art forms like poetry or novel writing. Film editing is often referred to as the "invisible art"^[1] because when it is well-practiced, the viewer can become so engaged that he or she is not even aware of the editor's work.
On its most fundamental level, film editing is the art, technique, and practice of assembling shots into a coherent whole. A film editor is a person who practices film editing by assembling the footage. However, the job of an editor isn’t simply to mechanically put pieces of a film together, cut off film slates, or edit dialogue scenes. A film editor must creatively work with the layers of images, story, dialogue, music, pacing, as well as the actors' performances to effectively "re-imagine" and even rewrite the film to craft a cohesive whole. Editors usually play a dynamic role in the making of a film.

Film editing is an art that can be used in diverse ways. It can create sensually provocative montages; become a laboratory for experimental cinema; bring out the emotional truth in an actor's performance; create a point of view on otherwise obtuse events; guide the telling and pace of a story; create an illusion of danger where there is none; give emphasis to things that would not have otherwise been noted; and even create a vital subconscious emotional connection to the viewer, among many other possibilities.
There are several editing stages and the editor's cut is the first. An editor's cut (sometimes referred to as the "Assembly edit" or "Rough cut") is normally the first pass of what the final film will be when it reaches picture lock. The film editor usually starts working while principal photography starts. Likely, prior to cutting, the editor and director will have seen and/or discussed "dailies" (raw footage shot each day) as shooting progresses. Screening dailies gives the editor a ballpark idea of the director's intentions. Because it is the first pass, the editor's cut might be longer than the final film. The editor continues to refine the cut while shooting continues, and often the entire editing process goes on for many months and sometimes more than a year, depending on the film.