Watch this YouTube clip of Bette Davis appearing on The Tonight Show with Johnny Carson to see what she thought of working with Faye Dunaway. Fast foward to 9:50 to see what she says about Faye:
http://www.imdb.com/title/tt0074411/board/post/tt0074411
I just found this anonymous post on Data Lounge:
I seem to recall a pre-Mommie Dearest interview with Bette, where she slammed Faye for being completely unprofessional on the set of "The Disappearance of Aimee." In either the same interview or a different one, she praised Joan for always being professional despite their on set clashes. Bette, I believe, had more respect for Joan than for Faye and would've happily worked again with the former than the latter.
James Woods, who co-starred with Bette and Faye on "Aimee," singled out Faye in a 1979 People magazine interview as the most unpleasant person he had ever worked with.