Installment II laying out the ideas from Thomas McCormack's classic about editing.
Diagnosis requires Sensibility and Craft
One difference between a private reader, whose responses can be right on, and an editor should be this: The editor ought to be able to identify what is causing the response. The private reader may be unengaged, deflated, frustrated or baffled by a book—but he probably can't specify what events, passages, lines, very words, are making him so. If the first stage of the editor's examination (his reading and responding) uncovers symptoms (undesirable effects in the then and only then should he go on to the second stage—diagnosis. Diagnosis entails the use of technical tools and tests to track back from the symptoms to the faults that are causing them.
The very term diagnosis should be accompanied by the maxim First, do no harm. Applying technical diagnostic tools to a well manuscript tends to lead to treatment, which can only harm a healthy specimen. But assuming the script is not perfect, the assignment now is to specify why not. First express the symptoms—the undesirable feelings, impressions, convictions the reader has. Then pinpoint what in the script (or missing from the script) is causing them. Then stroll on to treatment - see part Three.
Authors frequently gripe about editors, "When he talks to me about what's needed im my manuscript, he's rambling and vague". The reason for this is that—despite possibly having an adequate sensibility—he has no craft.
To see what fiction-editing craft might be, start by looking at the faults it's intended to detect. There are two kinds: surface faults and internal faults. A surface fault is local, as immediately evident to the naked eye as a skin blemish. They include failures of diction, grace, freshness, materiality, credibility, pace, vividness, understandability, interest. You can point at specific words that constitute it. Most surface faults do not produce delayed reactions. Because these components are on the surface, don't require special craft to detect.
Rarely should a book be edited in one reading. The largest, the most basic faults, the internal failures, don't betray themselves with one smoking phrase. They don't, in general, lie on the surface of the novel. They can't be recognized at once as faults. Often, indeed, the problem is that something has been omitted entirely. And sometimes they are composites, an element becoming a flaw only because a succeeding element doesn't consummate. This means you can't possibly recognize it as a flaw when you first read it" A "promise", tacitly made by the author to the reader early on, can be an enhancing enticement; but if it is thereafter ignored, not resolved either by fulfillment or surprising, justified reversal, it becomes a flaw in the book. What qualifies it as a flaw is that ultimately it leaves an unwanted negative feeling in the attentive reader. Irrelevance is in effect a failure of promise.