Red Superfly
You creepy little stalker
Guys watch the DVD commentaries. Lucas says that Vader was originally written as a thug who killed Jedi and was kinda 2D.
Also, check out one of the original Star Wars scripts. Darth Vader is nothing but a man, kinda like Admiral Piett.
Darth Maul is also what Darth Vader was originally going to be - an evil Jedi killing machine, who happened to have killed Luke's father.
In the gap between ANH's success and during ESB's creation, Lucas came up with the whole father angle for Vader. Vader's character is also changed - becoming more cool and collected, and being thrust into the limelight as a political player, rather than being the muscle for a Moff.
Lucas really did make things up as he went along. Some things he always had planned - The whole Luke and Leia being twins deal was part of the original script of Star Wars about the Skywalker twins, a boy and a girl.
Lucas didn't write the saga word for word. He had a general idea, and because of certain circumstances, he had to cut and edit his original script, leave bits out etc. Then there were things like Darth Vader, who's character evolved by itself, rather than Lucas knowing where it would go from day one.
It's always quite amusing that Darth Vader is a huge accident (the reason he became a "mechanical man" was because the limitations of ANH meant that he would be boarding a space ship, rather than an exotic planet first, and so needed a space suit, Ben Burtt gave him the breathing sound - and only by ESB did Lucas come up with the idea he NEEDED the suit at all times because he was badly hurt). I mean, it was also obvious that the name "Darth" also changed in its meaning. In ANH - Darth Vader is his NAME, because at the time of writing, his name WAS Darth Vader. Darth as in "John" and Vader as in "Smith". Then at some point "Darth" became a TITLE like "Mr." It's full of changes.