When we talk about writing honestly, we don't necessarily mean that the author is telling the truth. Stephen King, for instance, is (or at least was – some of us are a bit concerned about his more recent novels) an exceptionally honest writer. Yet we know every word is a lie. There really aren't cars like Christine that possess their owners, nor are there cosmic spiders disguised as clowns living in the sewers of Derry, Maine. In fact, there isn't even a Derry, Maine.
Where King's honesty makes itself apparent is in his handling of characterizations, observations, and human interactions. It doesn't matter that Stuttering Bill, the boy in It, never existed, nor does it matter that his brother, equally non-existent, was never murdered by a shape-shifting clown. What counts is that Bill's reactions to this murder, and to his parents' inability to cope with it, are honest reactions. They may not be the only responses possible; but they are honest responses nonetheless.
Some authors, however, write characters who feel, react, and respond in ways which are apparently dictated solely by the necessity of plot. For instance, I'm sure we've all read a murder mystery in which someone loses a loved one to a violent serial killer, yet is laughing and drinking champagne with the detective who solved the case 48 hours later? Does that seem real to you? It doesn't to me.
The same thing applies to non-fiction writing. When we write an essay, we're supposed to be putting on paper our thoughts about a particular subject. All too often, however, what passes for "our thoughts" are really other people's thoughts which seem clever to us. As a result, many essays are little more than a series of quotations from "experts" – acknowledged or otherwise.
For example, in an essay about the importance of a good night's sleep to our health, one student made reference to a sleep expert who said that you can't catch up on lost sleep. Now did the student really believe that? The first question that comes to my mind is, "What the hell does that mean?" If we never catch up on lost sleep, does that mean that the effects of that sleepless night I had five years ago are still with me? What about all those nights as a baby that colic kept me awake? No wonder we grow old and die – every hour of lost sleep is another nail in our coffin.
This isn't a matter of checking our facts, although that is also important. The point is that some "facts" automatically raise questions in the minds of virtually all those who hear it, yet few are honest enough to admit to it.
In fact, perhaps it is this ability to say "What does that mean?" that forms the foundation of honest writing. In It, the plot required Stuttering Bill's brother to be killed with the results that his parents retreated into a cold, emotionless shell. A more dishonest writer would simply have stated these things in one form or another. King, on the other hand, asked himself, "What does this mean?" "What does it mean to have your younger brother brutally murdered?" "What does it mean to have your father and mother withdraw their emotional support because they are too hurt and devastated to see beyond their own needs?" And then he wrote it.
In essence, this is the basis of good essay writing. We ask ourselves, "What does this mean?" and then we write it.