Diane Keaton’s portrayal of Kay was the second weakest of any actor in any of the three films. First, and she only appeared in the last, was Sophia Coppola. No fault of her own. She simply wasn’t an actress. Pretty good director, as it turned out, but she couldn’t even have been described as an amateur actress. Just the director’s kid. I felt sorry for the critical abuse she took. In Keaton’s case she was just horribly miscast.
The criticisms of Sophia Coppola were terribly unfair. She was the perfect Italian-American princess—a daughter, not a movie star—the second generation born in America and third generation living in America. Anyone expecting her to look like a Northern European princess or speak like a goombah's camare was off base -- her looks were on point, and her speaking voice and accent were typical of the third generation who were sheltered and privileged.
Diane Keaton's role in Godfather III was seriously undermined by whomever styled her hair and clothing. Both were ridiculous—the cheap brassy perm, the flashy satin fabrics, the gold colors—and entirely out of character. A woman of Kay Adams' background would have been wearing Ralph Lauren and have a sleek blow dry hairstyle. Not her fault; but most unfortunate.