Wondering what editors do for other authors? How about what recent Nobel Prize winner Alice Munro's three editors do for her? Sometimes, the minimal:
“We don’t have to do much,” laughs Close, the hint of a southern drawl in her voice. “With many of Alice’s stories, they come in and none of us touches a word. But every now and then there are stories she’s a little stuck on and one of us will give a suggestion that proves helpful.”—from "Editing Alice Munro" on Quill & Quire
But that's not to say that not taking action should be equivocated with not doing anything. In Munro's case, it's a given that the stories being submitted to these three editors (Anne Close, Douglas Gibson, and Deborah Treisman) are good. At that point, the assessment that needs to be made is whether there's any improving to be done. If they're already in top form, the editor's responsibility is to convince the author to leave it alone, which, given the infinite possible combinations of words an author can consider at any time, can be quite the chore.