An editor is someone who marries the intent of the script with the available filmed material to tell the best possible story. An editor's general goal is to find the most economical way to generate the biggest emotional impact in the viewer.
In so doing, an editor must watch every scrap of recorded film — often totaling many times the proposed running length of an episode —and select the best takes of each scene, then produce a "rough cut", which generally runs overlong and serves only to give an idea of the potential of the material. From this point, the editor will add completed CGI scenes and pick-ups. They will also work closely with producers and directors to refine the product until it is "locked", or visually completed. Then, the editor will send his or her locked version to various post-production artists, such as graders, musicians and ADR recordists. On very rare occasions, the previously "locked" episode will return to the editor for a final change, as reportedly happened with the Rose Tyler scene in Partners in Crime.
Though they generally adhere to the writer's suggested scene order, the director's notes, and the producer's wishes, they are not simply technicians carrying out instructions. They are a vital part of the creative team, and can often bring narrative dimension to a piece that other team members could not have foreseen. On some occasions, they may independently create versions which deviate from received instructions in order to demonstrate possibilities their colleagues had not considered.
Editors were initially relatively unimportant to the production of Doctor Who. For most of the 1960s, episodes were filmed more or less in narrative order as stage plays. Many early episodes do not have even a single edit in them; others have no more than one or two. Even in the 1970s and 1980s, a lot of what would today be accomplished through editing was done live-in-studio by the process of vision mixing. Thus, many 70s and 80s episodes employed only a "film editor", a term of somewhat ambiguous definition and inconsistent usage. At least initially, though, it was a narrowly-defined term which then meant "the person who edited footage captured on film". This was because throughout most of the 1963 version's history, location and special effects work was captured to film, while studio work was recorded on videotape.
Now, though, all footage is captured on the same medium. All footage thus comes through a single editing bay, making the editor a central figure in the hierarchy of post-production.