Mein liebster Feind - My Dearest Enemy (1999)

As a meditation by a director on an actor, it is unique; most show-biz docs involve the ritual exchange of compliments. My Best Fiend is about two men who both wanted to be dominant, who both had all the answers, who were inseparably bound together in love and hate, and who created extraordinary work – while all the time each resented the other's contribution.
https://en.wikipedia.org/wiki/My_Best_Fiend
In the 1950s, an adolescent Werner Herzog was transfixed by a film performance of the young Klaus Kinski. Years later, they would share an apartment where, in an unabated, forty-eight-hour fit of rage, Kinski completely destroyed the bathroom. From this chaos, a violent, love-hate, profoundly creative partnership was born. In 1972, Herzog cast Kinski in Aguirre, der Zorn Gottes (1972). Four more films would follow. In this personal documentary, Herzog traces the often violent ups and downs of their relationship, revisiting the various locations of their films and talking to the people they worked with.
http://www.imdb.com/title/tt0200849/plotsummary?ref_=tt_ov_pl

As someone with some directorial experience, this film resonates with me very deeply.