For me Ian McDiarmid's performance in the PT is the best thing in those films. He was clearly at great pains to slowly become the emperor. His voice subtely changes and the makeup changes too. It's clear from interviews and commentaries that he always thought Lucas was on the same page as him so was a bit miffed (He wouldn't put it so strongly cause hes an RSC pro) when all the subtelty was thrown out the window in favour of having Mace burn his face.

I grew up naturally assuming that the emperor looked mumified like he does in ROTJ because his soul was so rotten and corrupted by the darkside that it had even began to eat away at his body. Like the ring did to gollum.

But apparently no. The PT trilogy makes it clear that the only thing worshiping the darkside does is give you yellow contact lenses. sigh