Something that has always bugged me, though, is the apparent notion (or fact, since George talks about it all the time) that Vader's sole ambition is to overthrow the Emperor. I see very little in the OT to support that claim. His actions just aren't consistent with a man trying to kill his boss. In fact, he seems very much like the perfect subservient lapdog to me. The only "evidence" is his speech to Luke in ESB, but I've always interpreted that as a bluff to get Luke to join him. In ROTJ, it's even further established that Vader has no intention of going against Palpatine. Think about the scene between Luke and Vader. Luke does exactly what Vader supposedly wanted him to do from the last movie. Luke actually gives Vader the opportunity to "come with him." It may not have been under the terms that Vader wanted, but it was the closest shot he'd had. Luke was volunteering to ally himself with his father, and Vader just turns it down because he "must obey his master," instead of using the opportunity to bond with his son and team up with him against the Emperor like he apparently wanted.
And during the entire throne room sequence, Palpatine makes it pretty clear that he plans on replacing Vader with Luke, but Vader apparently doesn't mind being trash-talked because not only does he sit there and take it, he actively supports the plan, chasing Luke around and fighting with him just like the Emperor wanted. Are these really the actions of a man who plans on rebelling? Um... no. These are the actions of a whipped little man who would even sacrifice himself to obey what his boss tells him to do.