I have the Dorling-Kindersley Star Wars Visual Guide of the OT, which came out in 1997, and there it says the Emperor looked the way he did due to his lifelong immersion in the Dark Side.
After watching TPM I expected there to be a progression of his Dark Side corruption becoming more evident with each passing entry, and I was sorely disappointed when ROTS establishes that actually it was due to Mace Windu deflecting Force Lightning back at him. So stupid.
I recall the official SW website came up with some bullshit explanation that actually Palpatine was using the Force to project a non-corrupted looking version of himself. Yeah right.
I think it was a mistake bringing McDiarmid back for the prequels. No shade against his acting ability, but Palpatine really needed to have been played by a much younger actor to drive home the severity of his dark side corruption. James Marsters I feel would’ve been the perfect choice.
This could not be more wrong. He’s easily the best part of the prequels.
For what it’s worth, Lucas did try the gradual degradation thing. In Attack of the Clones he looks noticeably more pale and withered. It’s very noticeable in the “I love democracy” scene. For some reason in Revenge of the Sith this was dropped.