Fiction writers often have (or have had) rich lives in other fields, like Michael Crichton in medicine, or John Grisham in law. But they don't know a lot about everything they may have occasion to write about. For that reason, they do research--and when they are writing about something abstruse, they will run the relevant passages by an expert in the field. This practice is quite common--and also quite effective. All successful fiction writers are necessarily adept at the craft--at plotting and character and narrative development--but those skills do not come "at the cost of technical competence." Obviously, they can't know everything--and yet often a story may lead a writer into special areas, say into numismatics or into ornithology, with which he is unfamiliar. When that happens, as so many fine writers have done for decades and decades, he consults somebody who knows coin collecting or avian behavior. He wants to get it right.

The trick in writing really isn't "how much to tell of what you know"; rather, where necessary, it's in how to find out enough about what you want to tell. Good writers have excellent instincts that guard against over-writing. Trimming the excess is a key skill--and when they're lax, usually a good editor will do the trimming for them.