No historical character in a novel should do or say anything that you don't know he said and did. You can't displace him in time, and you can't move him geographically. And you've got to be true to him. If I wrote a novel that included Billy The Kid, it would be the Billy The Kid out of history; in other words, he couldn't be the main character....I would never quote Billy if I didn't have a valid quote. I wouldn't put him in any part of New Mexico that he wasn't in at that date; I believe you owe that to historical characters. Nothing distressed me more than to see an historical character in one of those historical, romance novels take the hero aside and give him a little advice on his love life or something. I don't think you have a right to do that with historical characters.
Shelby Foote (1916-2005), Conversations with Shelby Foote, 1989