When George Lucas started shooting Star Wars, no one knew exactly what Darth Vader was going to sound like. While shooting the film, the guy in the Darth Vader costume, David Prowse, was the guy delivering all of the the lines from the script, and it's pretty hilarious listening to him. There was nothing menacing or threatening about his west country English accent at all.
There are clips of the actor delivering his lines while shooting the film that were part of a Star Wars documentary that you may or may not have already seen. Either way, this video is still worth watching as a reminder of what the actors of this movie had to work with.
It is kind of sad that Prowse didn't know his voice would be replaced at the time, but let's face it... it was for the greater good.