The professorial dictum has always been to write what you know, but I say write what you don’t know and find something out.
In his recent essay featured in The New Yorker, writer T. Coraghessan Boyle discusses the act of story writing as “an exercise of the imagination.”
I don’t know what a story will be until it begins to unfold, the whole coming to me in the act of composition as a kind of waking dream…