The Emperor was originally intended to be a clever bureaucrat and politician by the name of Cos Dashit, later changed to Cos Palpatine, who became President/ Chancellor (the exact title was never made clear in the original drafts), proclaimed himself Emperor - perfectly legally - then left the day-to-day running of his Empire to his advisors, who were to be the real perpetrators of the heinous crimes committed by the Empire.
Darth Vader was originally conceived to be a character named Prince Valorum, and his design was slightly different, but otherwise he was essentially the same character. Tarkin was meant to be the leader of a crazy religious cult that worked with the Empire.
While Lucas has never commented on this publicly, to an historian's trained eye it seems that there were far more parallels to the Roman Empire and Augustus in the original drafts than the Nazis, which later became predominant. As such, the use of royal titles such as Prince, Princess, and Moff (which is apparently based on Turkish word for some kind of priest-soldier) made perfect sense; the Holy Roman Empire was more of a confederation of different principalities and bishoprics under the rule of a single Emperor, rather than a solid bloc as Nazi Germany was. Of course, Augustus had nothing to do with the later Holy Roman Empire, but I hardly expect historical accuracy from George Lucas.
EDIT: Wookieepedia finally decided to co-operate. Links added.