How Heinlein plotted

I'm powering through the ending of the smashing, enormous first volume of the first major authorized biography of Robert A Heinlein: Robert A Heinlein: In Dialogue With His Century, Volume 1: Learning Curve (1907-1948) and I've just been poleaxed by this quote, which absolutely sums up the way that I approach stories: "My notion of a story is an interesting situation in which a human being has to cope with a problem, does so, and thereby changed his personality, character, or evaluations in some measure because the coping has forced him to revise his thinking. How he copes with it, I can't plot in advance because that depends on his character, and I don't know what his character is until I get acquainted with him."