For non-writers, yet voracious readers, this has always been a question tantamount to asking where did the universe come from? The sources seem so mysterious, inexplicable and almost supernatural.
Veteran writers, on the other hand, writers who have been at it for years, approach the question from a different perspective.
I have found over the years that, whatever a writer’s aspirations/pretensions, he or she falls into one of two categories 1) calculating people pleasers whose intended bulls-eye is a bestseller list and revenue, and 2) writers who approach their tasks humbly but with a sense of adventure as if embarking on a journey in pursuit of some essential irreducible human truth that cannot be advanced upon directly, but only stalked through the lives of characters in certain situations of duress.
No surprise, of course, that the bestseller lists are populated by the people pleasing, generally plot-driven sort of writers. But their fame and glory is short-lived.
Nor should we be perhaps be surprised that the books that outlast the whims and fads of the moment are written by the quiet stalkers of their characters’ motives, situations, and, like hunters, follow them through the forests and underbrush of their lives, closely, ever so closely, to see what will develop, what will happen next that has the ring of “truth,” and what that truth suggests for us as human beings.
If you are interested in how these more burdened, yet more serious writers think and operate, I can refer you to the best source currently available to inquiring minds: The Archive of the Paris Review Author Interviews. That’s the place to look if you are truly curious about the ground from which great books spring.