(Khoty Mathur was saying that I ought to write a piece on editing to communicate how important it can be for an author - on the strength of my having 'edited' Sirens Spell Danger. Do not know how well this piece serves that purpose)
I have felt like saying, "That looks like no tree I have ever seen" when I looked on a painting. I have had the wisdom keep my mouth shut because I have realized someone would say something like, "That is a Cubist painting not a #@$% landscape". I have felt like saying, "No-one has a nose that long" but have known someone would say, "That is a freaking cartoon not a damned portrait." That element of wisdom was there because, like most arts, painting is something that is not a part of your normal routine unless you make it so and, thus, it was easier to 'know that you do not know'.
Writing and reading, though, ARE a part of my regular routine. That, in effect, means that I feel competent to be an author because I know writing, and to be a reviewer because I know reading (Reviewing as in saying something more than 'I did not like this book'). I do not feel lonely any longer, though, because I have discovered I have a bit of company in the first illusion and a lot in the second. The supreme folly - of considering an experience of reading as sufficient to edit a book - I reserved for myself.
The problem with editing is that you cannot, like a reviewer, taste rum, spit it out and call it bad quality only because it does not taste like wine, and you like only wine. Even if you knew enough to restrict yourself to wines, you cannot disparage a sherry for not tasting like champagne. You have to see whether the rum (or the sherry) is good AS a rum (or a sherry) and, if not, tell the author what is missing and, ideally, what needs to be done to make it good. To revert to the painting example, one needs to tell the Cubist something like, "I think that bough actually looks like a bough, which is spoiling the effect. Maybe you should try to make it look like an octopus tentacle shooting flames" and the cartoonist, "This will look better if you make the ears pointed and add a couple of horns."
Editing involves a lot more than merely copy-editing - to check grammar, redundancies and monotonous repetition of words - and Proof-reading - to rectify typos and punctuation errors. A good editor actually should identify, if not suggest means to rectify, plot logic errors; suggest modifications in narrative structure to enhance the effect; check for narrative consistency; verify whether the book gets boring in parts and suggest corrective measures; and check if what the author intended telling will be clear to the readers. All of this needs to be done and modifications suggested while maintaining the style, tone and basic story idea of the author intact. If, as an author, the emendations required in your book are more than the size of the book, you may either get the book thrown in your face or blood all over the book when the editor shoots himself in the head.
The basic genesis of the story is a story idea. To illustrate from the 'best' story in the book I edited, the story idea would be "A molested young girl - forced into it for money by her father - takes revenge on her molesters". You can conceive of multiple plots for the story, involving why her father forces her into this situation, the way she takes revenge, what happens to her at the end etc. The exact set of incidents dreamed up by the author would be the plot. You could have a good plot with an unoriginal story idea OR a great story idea can be messed up by a bad plot AND, as an editor, you need to know which of the two has happened or if everything is fine/messed up.
You can conceive of a further change by the manner in which you stress different things. For example, you could write of what is going on in her mind in great detail and the murders as a way in which she is trying to find closure. You could make it a revenge drama, with the police closing in on her and write in enough of her mental trauma to make the reader want her to succeed before she gets caught. Or, you could go at it from the investigative point of view, and make her identity and motives the twist in the story. So, with the same plot, you have various stories that can be written.
Even given the plot and the sort of story you intend to tell, the story can read differently depending upon the order of the incidents. You could write everything from the beginning to end. You could write the story with her starting her murders and, for each murder, go back to what happened with that victim and, then, the way she kills them. Change the order in which you tell things and you change the story that is getting told. If a tale is boring, it could well be merely because the narrative structure does not make it interesting, while an alternative could. So, an astute editor can, sometimes, rescue a story without junking the plot by suggesting changes in narrative structure.
The rest depends on technique - choosing the right point-of-view characters, making the characters believable, the narrative flow smooth etc. The thing to note, though, is that if you conceive a plot that requires characters to do some things at various times, the characters ought to be such that they can believably do those things OR believably change over the course of the story to be able to do them. You cannot have a rank amateur at espionage change over two days into a Dead-eye Dick who can storm a citadel of terrorists shooting down a man with each shot. Movies seem to have the liberty, novels do not.
So, what has all this to do with editing? A good editor needs to see exactly what sort of story the author is trying to tell, and see if the way the author is saying it makes the story credible and interesting. The author may just write without conscious cognizance of the elements of his writing. The editor has not only to read the story to see if it appeals but has also to see what changes are needed to make it appealing OR enhance its appeal. This means that the editor HAS to be able to parse the story to check out the appeal of the story idea, the appeal of the plot and narrative structure, the believability and appeal of the characters, and the efficacy of the narrative. The great difficulty is in setting aside your own personal tastes in stories, types of characters and narrative styles.
This, then, means that the editor should point out whether and why a story is not working and, ideally, the suggest changes in the WAY the author writes to make the story workable; if THAT is not possible, then change around the narrative structure to make it a more workable and interesting story; if EVEN that is not possible, then see what portions of the plot needs to be modified.
The process actually involves getting INTO the author's world; see how the author WANTS to tell it AND help him do it. An editor is not good if he edits a tale of 'literary fiction' and suggests changes that will modify it to 'pulp fiction' OR the vice versa. The result MAY be a better story but it would NOT be the author's story.
Of course, at the end of it all, the copy-editing and proof-reading HAS to be done.
THAT, in short was the job I took on when I chose to edit "Sirens spell danger" - instead of merely putting my name as editor and leaving it at that. The result WAS a lot of heartburn from my co-authors, I suppose, because of the rewrites I was forcing on them BUT I sure hope that the stories still remained the respective authors' stories and were no worse for my intervention.
One thing the process taught me, though. A good editor (not me :) ) can make a great difference to a story - and not merely by way of clean copy. It, incidentally, also taught me that the editor's job, in addition to being thankless, is a lot more painful that I thought it would be.