Today's, from The Paris Review interview with the late Christopher Isherwood, speaks to turning a real person into a fictional character:
Good things grow from details...
When you’re writing a book, you ask yourself: What is it that so intrigues me about this person -- be it good or bad, that’s neither here nor there, art knows nothing of such words.
Having discovered what it is you really consider to be the essence of the interest you feel in this person, you then set about heightening it. […] trying to create a fiction character that is quintessentially what you see as interesting in the individual, without all the contradictions that are inseparable from [the] human being, aspects that don't seem exciting or marvelous or beautiful. The last thing you're trying to do is get an overall picture of somebody, since then you'd end up with nothing.