The one thing I HATED about the absolute EPIC Godfather movies was the moronic Diane Keaton character. She played a whiny simp of a woman who I just couldn’t stand.
I couldn't stand the character (Kay), either, but Keaton must've played the role just as Coppola wanted her to. This site claims Mia Farrow, Blythe Danner, and Michelle Phillips (of the Mamas and Pappas) also were considered for the role.