Take a moment to watch this video from Star Wars on set footage. In it you can hear the original voice of actor Dave Prowse, who played Darth Vader, the voice was later replaced with that of actor James Earl Jones.
Whilst Prowse, also the Green Cross Code man, had the size and physique for Vader, the West Country accent does not quite lend itself to the menacing tones of one from the Dark Side.
Step in James Earl Jones to supply the voice talent and the great work of post production sound for creating the sound of Vader we all now know.
It just goes to show that sound is just as important as picture in establishing iconic movie characters.