As a kid I saw "Aguirre La Colere de Dieu" on TV and was terrified by Kinski. But after all, what counts is only the film, and on movies you never see the director. Now if you watch "Mein liebster Feind - Klaus Kinski" at last you can see the other side - Werner Herzog. Or have glimpse of it.
Because of course, everything is from Herzog's point of view, so in a way this documentary does not differ from the real movies themselves : you see Kinski acting like a madman in front of the camera or not. Shouting at people. One terrifying scene is an excerpt from Fitzcaraldo where Indians seem to complain about the character played by Kinski, while Herzog is commenting that those very Indians had offered to kill the actor...
Then you've got comments by other people, particularily actresses who witness about Kinski's other side, shy, caring. And the final scene with the butterfly - something Kinski the actor could do, but Aguirre could not. It seems as if Herzog, having showed only one side of Kinski all these years, tries to explain it was all for the movies. Even if he still portrays him as a mad genius (and coward at that), his selective memory tries only to remember the good times, a few rare footage of Kinski and himself having a good laugh.
What the documentary also stresses out is that the director and the actor could have been switched during the filming of Fitzcaraldo : Herzog explains how he was alone against everybody, believing the impossible, like the character in the movie. At this point only, Herzog seems to aknowledge the fact that he also was completely mad.
I recommend watching the documentary, and then watching Cobra Verde, Fitzcaraldo and Aguirre again.