"Prowse spoke his lines in his West Country accent during the filming in London as Carrie Fisher playfully nicknamed him "Darth Farmer" because of his rural tone. He said he realized his voice was hard to make out through the thick mask but was told his dialogue would be added down the road. But months later, back in the U.S., Lucas subbed in the deep voice of James Earl Jones for his during postproduction. "
I’d love to see a cut of Star Wars with Prowse doing the vocals. I bet it would be a riot.