Difference between PT vs OT Vader

[Darth Cronos the Proud]

It's pretty obvious that between ROTS and ANH, Vader undergone a distinct transformation in his character from "Whiny-Anakinish" Vader to Darth Vader, "da Man" that we all have come to known for a long time.

Most interestingly, I have observed the change in relationship between Vader & Palpatine.

In ROTS, Anakin/Vader & Palpatine were very close friends. They had a mentor-student relationship and Anakin/Vader had... dare I say a reverence for Palpatine? (At least this is what I got when watching the PT).

However, by the time of OT-era, Anakin/Vader is clearly plotting to overthrow the Emperor, evidently by his attempt to convert Luke and rule the galaxy as "Father & Son".

So why this change in relationship between Vader & Palpatine? I know it is the Sith way for the student to destroy the Master. But did anything happen in between? Perhaps those with extensive EU knowledge can shed some light...

I think one of the major reasons for Anakin/Vader's transformation in the years between RotS and ANH is an increased understanding of the Force. When I watch RotS and ANH back to back, I get the feeling that the ANH Vafer is a lot more spiritual and "into" the Force. In the prequels, I see Anakin as believing in the power of the Force, but only in a vauge and uncertain sense (remember how he rants, in a somewhat pleading and very whiny way, that he could, with a proper understanding of the force, do anything), while in ANH Vader is extremely confident in the powers of the Force, and trusts in it completely (remember the Death Star round table meeting with Vader's awesome quote "Don't be so proud of the technological terror you've constructed. The ability to destroy a planet is insignificant next to the power of the Force.")