The key to this documentary occurs about half way through, when Herzog talks about Kinski's autobiography. Before this scene, all of the stories have been about Kinski's madness, his rage, his anger, how frustrating it was to be around him. After that scene, the human side of Kinski is revealed - his compassion, his belief in Herzog as a film maker, his severe germ-phobia. The reason the autobiography scene is key is because it reveals that much of what Kinski and Herzog say about each other to the press and public is fabricated. Or, not fabricated, exactly, but not the whole truth. In the scene in question, Herzog reveals that he and Kinski wrote parts of Kinski's autobiography together - all the stuff in there about Herzog being a vile, despicable human with no redeeming qualities where written by Herzog and Kinski, sitting under a tree. The first half of this documentary is just repaying the favour - Kinski understood that his madness was what attracted the public to him, and was alright with that.
But whereas Kinski's autobiography (according to this documentary - I haven't actually read it) never said anything nice about Herzog, Herzog's documentary - by revealing the semi-fiction of their relationship - does reveal nice things about Kinski. He was a mad egomaniac, he did always need to be the centre of attention, he would rant and rave and burst into violence with little or no provocation. But he could also be gentle. And kind. And human.
Another interesting thing about this documentary is that it raises questions of how much of Kinski's mad behaviour was an act. How much of it was, "well, this is what the public wants..."? Herzog talks about Kinski's faux-love of nature - how he wanted people to think of him as in touch with nature and the jungle, despite the fact that he actually hated it. How he would talk about nature as 'erotic', despite never touching it, never going near it. How much of Kinski's other behaviour was like this? Was it all an act - a put on - a lie? Did he scream because he needed to, or because other people wanted him to, or because he perceived that other people wanted him to? I guess we'll never know for sure.