I think in any writing you're paying attention to detail.
In nonfiction, you have that limitation, that constraint, of telling the truth.
I used to distinguish between my fiction and nonfiction in terms of superiority or inferiority.
When I'm in the field, when I'm working, I keep very careful notes. I wear big shirts with big breast pockets, and I carry in them two little spiral notebooks.
Here I am, safely returned over those peaks from a journey far more beautiful and strange than anything I had hoped for or imagined - how is it that this safe return brings such regret?
In fiction, you have a rough idea what's coming up next - sometimes you even make a little outline - but in fact you don't know. Each day is a whole new - and for me, a very invigorating - experience.
I was just very interested in the American frontier and the growth of capitalism - those enormous fortunes that were being made, more often than not, on the blood of poor people, black people, Indian people. They were the ones who paid very dearly for those great fortunes.