Gangrey.com links to this story about nonfiction narrative writer Lawrence Weschler talking about teaching nonfiction narrative at New York University, and saying:
"...it is less a class about reporting methods than it is about the fictional methods that can be applied to nonfictional writing. It presupposes that the writer will try to be fair, but also acknowledges that there is no such thing as objectivity, and revels in that fact. Then we get down to business and talk about all the stuff that’s interesting: form, freedom, irony, voice, tone, structure."
I would propose he could be a bit more rigorous than "It presupposes that the writer will try to be fair."
I'd suggest: "It demands the writer be fair and tell the truth -- not only report things that actually happened and things that were actually said, but portray them in context to tell a true story."