To my mind, the difference is clear. It's Lucas.
I agree. I felt so sorry for Natalie Portman in Episode III. "Oh Ani! YOU'RE BREAKING MY HEART!" Seriously. I cringe. That is no one but the director.
Watch Natalie Portman in Black Swan. She is utterly brilliant.
Not that I hate Star Wars. Far from it. The broad story arcs echo many ancient myths and stories. George Lucas has said that he read Joseph Campbell's "Hero with a Thousand Faces" which surveys many ancient myths for commonality. The idea of a young boy escaping his mundane life to find his "hero's journey" is primal, as is the story of a son of facing the sins of his father. In my mind these things elevate Star Wars into the pantheon of cinema, in spite of the horrific dialog.