Star Wars Episode III: The Chosen One Featurette
At 2:34, George Lucas has this to say:
Darth Vader became such an icon in the first film, Episode IV, that that icon of evil sort of took over everything, much more than I intended. If it had been one movie, that wouldn’t have happened. He would have been revealed to be this pathetic character at the end of the movie. But now by adding Episodes I, II, and III, people begin to see the tragedy of Darth Vader, as what it was originally intended to be.
Let me emphasize that: Lucas refers to Vader being "revealed to be this pathetic character" as his original intention.
Vader was never meant to be cool. Vader was never meant to be “badass”.
Vader was, from the beginning, meant to be pathetic.
Lucas goes on to say:
And I like the idea that the person you thought was the villain, is really the victim. And that the story is really about the villain trying to regain his humanity.
Which essentially sums up the entire purpose of the Prequel Trilogy: Not to showcase Anakin Skywalker as a dashing hero, but to reveal the victimhood, the weakness, the humanity behind Darth Vader.
At 11:42, Lucas comes back to this point, saying:
Obviously one of the key elements in Episode III is that we see him actually put into the iron lung, the mask finally close on him, that now he’s an artificial man. But we know what’s inside of it. And I think there’s a new part to IV, V, and VI that people haven’t seen yet. The man in the suit comes into being a more personalized character than what he was originally.
I’ve often said that personhood is a major theme of the Star Wars saga as a whole and the Prequels in particular. (“I’m a person, and my name is Anakin.”) Within the story, Anakin becomes more and more dehumanized and depersonalized the closer he draws to the dark side, but is eventually able to regain both his humanity and his person, his true self, through the redemptive love of his son. From the audience’s perspective, we see at first only the mask - the cold, inhuman exterior of Vader within which the person of Anakin is all but lost. We catch a glimpse of the real man behind the mask at the end of Episode VI, but only through the Prequels, through seeing how he came to wear that mask in the first place, can we truly understand him. Thus, ironically, as Anakin loses himself, his self is revealed to us.
The featurette gives Hayden Christensen the last word on the Prequels:
…and it makes him more human in a lot of ways.