I've worn both hats--as a freelance magazine writer and as editor of a technical magazine and a variety of specialty publications. The jobs are quite different.
A good writer researches the piece for facts, gathers quotations and anecdotes for interest, organizes all the material into a structure that will create and maintain interest, writes a first draft to the best of his or her ability, and then (since this is a good writer) analyzes what has been written, makes notes on how to improve the piece, and writes a second draft, copyedits it, and sends it to the editor.
A good editor respects the writer's talent (the editor probably hired the writer in the first place) and all the work the writer has done to this point. That means to me that any editing that must be done should preserve the writer's intent, structure, and voice. When I am editing the work of a good writer, I hope and expect to change only typos and misspellings. Unless something is glaringly wrong, something has been included that should be left out or left out that should be included, I simply do not mess with the writer's words.
On the other hand, when I was editing annual publications for events such as the Transpacific Yacht Race, much of the copy I received came from people who were sailors not writers. I did a lot of rewriting, while still trying to preserve the voice of the writer and the good stuff he or she had written. I got notes of appreciation from them.
The best editor I have ever had is Tom White at Documentary, the magazine of the International Documentary Association. The biggest dispute we ever had was over the use of apostrophe-s in the singular possessive of a noun ending in s. I show the possessive as xxxxxs's, which is grammatically correct. He changed it to xxxxxs', which is grammatically incorrect but proper journalistic style (in some circles). I complained. He insisted. I lost. You can't get too upset at an editor who lets your language stand exactly as you wrote it, even where I pulled no punches in a couple of scathing reviews of dreadful, unreadable books by academics.
The worst was an associate editor at Video Systems magazine, who seemed to think editing was a chance to do a little writing on his own without the bother of doing research and interviews. I once wrote a piece for the magazine about finding and using stock footage in videos. In the second paragraph, I wrote a couple of sentences about a documentary I scripted on the Battle of Midway in 1942. It was obviously a situation in which one would need to find stock footage of American and Japanese ships and planes. When I got back the editor's version, I was surprised (appalled) to see that my article now opened with three paragraphs about the history and importance of the battle and didn't mention stock footage until late in the fourth paragraph, 250 words into the article. I sent an e-mail to the editor-in-chief, along with the manuscript with the associate editor's gratuitous changes highlighted in red--there were a lot of them--and got back a revised edit that was pretty much the same as the draft I had originally submitted.
When the editor-in-chief moved to another publication, and the associate editor became managing editor, I stopped writing for that magazine.
Why do some people become editors while others remain writers? I suspect the prime reason is freedom vs. security. A freelance writer doesn't get paid until an article idea has been accepted, researched, written, and approved, but can work from home in his pajamas. But some people don't like to live like that. An editor at a publication has to go to the office, but gets a regular paycheck.
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