Truth and fiction, and literary truth, or relevant truth, has always been an interesting topic to me. I don't know why Capote would say it's non-fiction if it's not, but he struggled a lot of health and mental wellness more than he put out to the public, so I don't want to speculate on it. It would have been far more interesting if he truly opened a discussion of truth vs. fiction.

My absolute favorite example of this is from one of my favorite authors, Tim O'Brien, in The Things They Carried, where he discusses multiple times the difference between real truth and story truth, and what they mean, and which is more true: the actual events that happened, or the meaning of the story provided by an account which isn't exact, but captures more of what's important.

In any war story, but especially a true one, it's difficult to separate what happened from what seemed to happen. What seems to happen becomes its own happening and has to be told that way. The angles of vision are skewed. When a booby trap explodes, you close your eyes and duck and float outside yourself. .. The pictures get jumbled, you tend to miss a lot. And then afterward, when you go to tell about it, there is always that surreal seemingness, which makes the story seem untrue, but which in fact represents the hard and exact truth as it seemed.

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A true war story is never moral. It does not instruct, nor encourage virtue, nor suggest models of proper human behavior, nor restrain men from doing the things men have always done. If a story seems moral, do not believe it. If at the end of a war story you feel uplifted, or if you feel that some small bit of rectitude has been salvaged from the larger waste, then you have been made the victim of a very old and terrible lie. There is no rectitude whatsoever. There is no virtue. As a first rule of thumb, therefore, you can tell a true war story by its absolute and uncompromising allegiance to obscenity and evil.

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A thing may happen and be a total lie; another thing may not happen and be truer than the truth.

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That's what fiction is for. It's for getting at the truth when the truth isn't sufficient for the truth.

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And in the end, of course, a true war story is never about war. It's about sunlight. It's about the special way that dawn spreads out on a river when you know you must cross the river and march into the mountains and do things you are afraid to do. It's about love and memory. It's about sorrow. It's about sisters who never write back and people who never listen.

It's hard to summarize his points throughout the book without reading it, because it's all important to form this point, but a nice piece is from this review written here

If we begin to notice certain inconsistencies in a novel, does this inevitably destroy its integrity and value? O’Brien himself says that “story-truth is truer sometimes than happening-truth.” Does this mean that a story written three or four times in three different ways can be true all three or four times? Or does it mean that a story is never true, due to the fact that it is recounted after the original event? Rather than choosing one or the other, perhaps we can conclude that these two premises are not as contradictory as we might think.

Through a close examination of the multiple-perspective method of storytelling, and the resulting ambiguities, of one pivotal event in The Things They Carried, it becomes apparent how O’Brien’s novel supports both of these premises. Paradoxically, although common sense might suggest that the use of this kind of ambiguity would contradict the truth-value of the book, in O’Brien’s case, it is a necessary component that actually serves to strengthen it. One of the key incidents that demonstrates this is the death of O’Brien’s friend and fellow soldier Kiowa.

Represented in various ways across four chapters, the story of Kiowa’s death remains ever mysterious and changing, and no one seems to be able to find the words to tell it properly. In the chapter “Speaking of Courage,” the reader is shown, through the post-war musings of Norman Bowker, the grim death of Kiowa on a rainy night in a Vietnamese sludge field. In “Notes,” O’Brien writes of how he was inspired to write the story and talks about the process that he went through in doing so, and in “In The Field,” we see Kiowa’s squad searching for his body the morning after his death. Finally, in “Field Trip,” O’Brien travels back to Vietnam to leave Kiowa’s moccasins at the place he died.

The only element strictly common to all of these accounts is that Kiowa remains dead at the end, and as the event continues to shift shape before the reader’s eyes, it is easy, and perhaps even natural, to begin to doubt the value that we might have otherwise placed in it. We may begin to feel that O’Brien is cheating us somehow or at least that he is not playing according to the rules. If we dig deeper, however, we can see that, even though he may have changed the rules to suit his purpose, we are still able to determine what the new rules are.

A quote that seems to fill well for that article as well:

A good piece of fiction, in my view, does not offer solutions. Good stories deal with our moral struggles, our uncertainties, our dreams, our blunders, our contradictions, our endless quest for understanding. Good stories do not resolve the mysteries of the human spirit but rather describe and expand up on those mysteries.

Over the next five years the committee members were murdered in a series of elaborate and gruesome executions. The first two victims were a lawyer and his wife. Climbing into their car one morning, they were attacked by nine huge rattlesnakes which had been placed inside overnight. The snakes had been injected with amphetamines to make them more aggressive and the victims' heads had swollen and turned green, Capote wrote, "like Halloween pumpkins".