Here's the thing though - in the originals, the acting was, for the most part, pretty damn good. The actors may complain, but they did a GREAT job. Alec Guinness, Ford, Fisher, Jones, even Hamill was great (in his role). But in the prequels, everyone except McDiarmid was horrible. Was it the blue screens, as Ewan attested to? Maybe.
Even McDiarmid was bad in ROTS. But in fairness, I think he intentionally chose to just give up on really acting and enjoy the paycheck. You can tell tell he makes a mental switch right at the "I am the Senate" line, which sounds so unnecessarily ridiculous. I imagine that this conversation happened:
George "Okay, so Ian, this is the scene where the Anakin converts into Darth Vader."
Ian "Yeah. I figured I'd read it ahead of time so I could mentally prepare."
George "The emperor in this scene has to be really evil. He's been hiding his true nature for years, but now he can finally show it."
Ian "Yeah. Like, you put the makeup on where we see his true evil physically revealed, and I think that's a physical metaphor for his emotional state."
George "The makeup means he's evil, right. So for this scene, I was really thinking about old monster movies like Frankenstein when I wrote it."*
Ian "Hm. I never really thought of the Emperor as just an all out monster. I mean, he's monstrous emotionally, but his manner is very sophisticated."
George "He was pretending to be sophisticated before."
Ian "I was just thinking about how in Jedi where you could see his hate burning beneath the surface, and before he shocked Luke with the lightning he was almost scarily polite."
George "Ummm.... We're getting off topic here. This is the prequel trilogy, they're different movies."
Ian "They certainly are."
George "Hm?"
Ian "Sorry, just talking to myself."
George "Okay. Anyway, just make sure Palpatine is really evil. His voice should be really deep, but have a sinister snarl to it."
Ian "Doing that requires two completely different vocal techniques. I'd like to choose one of them and really develop it."
George "No, it will work. Like, when you say 'Once more the Sith will rule the Galaxy!' make sure it's super evil sounding. But earlier in the scene, when he's talking to Anakin about cheating death, it should sound really even and creepy."
Ian "So basically, I should talk in a different voice depending on the moment."
George "Exactly!"
Ian "And you really just want me to play up how evil the character is."
George "Now you're getting it."
Ian "Alright George, I'll take care of it. Thank god I'm an established Shakespearean actor and this won't possibly ruin my reputation."
George "What?"
Ian "Sorry, talking to myself again."
George "Oh, okay."
I should mention though that the scene where he tells the Darth Plagueis story, even though it kinda doesn't make sense on a number of levels, is one of the best scenes in Star Wars, purely because of McDiarmid's acting.
----------------
*95% true.