After Anakin defeats Count Dooku aboard the Invisible Hand in Revenge of the Sith, Palpatine tells him to execute the defeated Sith Lord. Anakin hesitates because Dooku is unarmed and no longer capable of defending himself. Palpatine then drops the calm voice he normally uses as Chancellor and gives the order in a much deeper tone:
“Do it.”
Ian McDiarmid has never said that George Lucas gave him a special direction for those exact two words. However, he has explained how he created Palpatine’s harsher voice and why it sounds so different from the voice he uses while pretending to be a respectable politician.
During the 40th-anniversary panel for Return of the Jedi at Star Wars Celebration Europe in 2023, McDiarmid discussed the first time he saw himself wearing the Emperor’s makeup. His reaction immediately shaped the character’s voice.
“He looks disgusting, so I thought he should sound disgusting.”
McDiarmid began experimenting with the noise a person might make just before vomiting. He lowered it, extended it and turned it into the Emperor’s deep, damaged-sounding speech pattern. He jokingly described the final result as “a voice on the vomit.”
The voice was first developed while McDiarmid was preparing to play the Emperor in Return of the Jedi. After spending several hours in the makeup chair, he looked at the character and thought the Emperor resembled a “horrible, slimy toad.” He decided that a normal elderly voice would not match the face created by the makeup team, so he made the character sound physically corrupted as well.
McDiarmid originally believed that his voice might be replaced during post-production. George Lucas instead approved the performance and kept it in the finished movie. That decision established the voice later associated with Darth Sidious throughout the prequel trilogy.