Phyllis Rose (author of Parallel Lives and The Year of Reading Proust) makes some very interesting observations on the "expository style" in both essays and fiction, in a review of The Best American Essays 2004, edited by Louis Menand:
The observation has been made for decades that good nonfiction employs techniques of fiction, especially narrative. When we encounter a terrific nonfiction writer, such as Laura Hillenbrand, who can make even a racehorse interesting, we say she’s a great storyteller. But it’s equally a gift, the gift of the essayist, to see stories as examples of a larger idea. . . Nonfiction as artful as Seabiscuit doesn’t get written without the essayistic gift of marrying instance to abstraction. . .
Whether the result is nonfiction or fiction, certain writers move up and down the abstraction scale at a unique pace and with a unique pitch. Voice, a quality much prized by writers and connoisseurs of writing, as Menand points out in his astute introduction, is hard to define and impossible to create on demand. Nevertheless, we respond to it. Susan Sontag sounds like Susan Sontag whether we read the essayistic Illness as Metaphor (1978) or the novel The Volcano Lover (1992).
Where essayists who want to write novels can go wrong is in believing that, in fiction, they have to leave the expository part of themselves behind, just showing, not telling. In doing so, they silence part of their literary uniqueness. George Eliot made the transition from critic to novelist—a transition she wasn’t at all sure she could make—because she found herself able to imagine dramatically. But the transition worked as well as it did because she felt free to bring into the novels the same expository voice she had used in criticism. . . .
Thus, Rose seems to be suggesting, "story" in nonfiction ought not to be an end in itself, but rather a strategy used to communicate "a larger idea." Presumably the writer who does not do this is simply appropriating a "literary" technique that can't by itself bear the burden nonfiction would place on it. And while I would agree that an expository style--as opposed to a self-consciously "poetic" or descriptive style--is a perfectly legitimate way of writing a certain kind of fiction, I would also argue that when it is incorporated into fiction it ceases to function in the purely discursive way it does in nonfiction. It becomes part--indeed, the irreducible part--of the imaginative fabric a work of fiction pieces together. The "Susan Sontag" of The Volcano Lover is not the Susan Sontag of the nonfiction--or if it is, the novel is simply a failure at a very fundamental level--but is also assimilated into the independent fictional world the literary work creates.