In the early 1960s, before the United States became deeply involved in the war in Vietnam, Art Hoppe did a series of columns for the San Francisco Chronicle in which he satirized the conflict (in what we would now say is a rather racist way) by describing it as an ongoing dispute between two leaders in hilly country — Hoo Dat Opp Dar and Hoo Dat Don Dar.
Taking these questions seriously is a key to understanding how to write powerful fiction. Who is that? Who is it really? Is he like me? Does he think the way I think and feel the way I feel?
Every writer of fiction is an amateur psychologist. (We will tiptoe lightly past Jonathan Kellerman, who was at one time a professional psychologist.) That is, every writer of fiction has a concrete and practical idea of who people are. To write fiction at all, you have to have some notion of what’s going on in the heads of your characters. Who’s that up there? Who’s that down there?
One important difference between good fiction and the other kind — perhaps the important difference — is that writers of good fiction have more insightful theories about the human psyche. They also know how to assemble their ruminations into good stories, but you can’t forge a good story using poorly imagined characters.
This is one reason why mathematicians and chess players often do their best work when they’re 20 years old (or even younger), while fiction writers often remain active and produce wonderful work when they’re over 60. It takes time to develop an understanding of what it means to be human.
Jim Aikin's Oblong Blob 
Forgive a tangent from the point of your post, but the irony is that these days it’s much more likely a young fiction writer will produce a seminal work than a mathematician, because the latter needs to build on the great amount of work that’s happened in the last few hundred years.