You needed someone physically imposing to play the part and David Prowse fit the bill perfectly – tall and muscular. (Prowse is a 6′7″ bodybuilder). He was also in great shape to stand up to the rigors of acting in a rather cumbersome costume.
However, his voice was terrible. Prowse’s voice his fairly high pitched (in the tenor range) and has a distinct West Country lilt to it. It didn’t sound scary at all.
They needed a voice with more gravitas and turned to James Earl Jones who has a deep baritone. He overdubbed all of Vader’s lines, which were originally spoken on set by Prowse. Jones didn’t even ask for a credit for his work as he thought the film wouldn’t draw much attention.
When they finally had to unmask Vader, there was another problem. Prowse is solidly built but also has a thin face. They went with Sebastian Shaw who has a nice sized head that better fit the helmet and the shape of Vader’s armor. He also has a nicer accent.