I came across this gem the other day, an apology letter from FFC to Marlon Brando written during the filming of Apocalypse Now. It details his thoughts for Brando's character, Kurtz (called Leighley at the time) with this great line:

'The interesting thing about this character is that he is whole, he is irrational and rational all in one, and that is what people are like.'

The whole letter is reproduced here:

http://www.lettersofnote.com/2011/01/we-will-never-get-past-viet-nam-if-we.html