Like any collection of stories, what is real and what is fiction become inextricably entangled. For example, I played with this band that burned down a prom in South Dakota. I started with that premise and went from there. Marquart said the truth in real events can often be brought out better when it is fictionalized. Many stories with disparate parts can be missing something until tied together with fictional elements that show us a deeper meaning underlying them.
Imagination is the key.
Sometimes when you write about things that really happened they are missing an element, so you have to fictionalize in order to explore elements of imagination in the reality of what happened. Marquart's first love is poetry. The satisfaction of writing poetry is more immediate while the rewards of fiction are more delayed. You can work on a poem in an afternoon and shape it into something that feels finished. Writing prose requires stamina, especially fiction.