Diane Keaton is a very famous actress known for her roles in the 1970s and 1980s. Aside from her appearance in Godfather, her early career consisted of being carried by Woody Allen. After putting up with his comedy gags as a nebbish, she had finally gotten away from him in the late 1970s.
Having picked up acting skills from classic ensembles, she is one of they few Hollywood actresses to bother reading the script. Jack Nicholson has noted that she is the only actress that memorizes the script before getting onto a stage.
And she plays the awkward cute girl like nobody else. La-di-da!