RIP Darth Vader 🖤Watch
A 6' 6" bodybuilder, he applied for the role of Superman, but finished up as personal trainer to Christopher Reeve. With various, though minor, acting roles, he was probably best known in the UK as the Green Cross Man in road safety adverts, until his size landed him the role of Darth Vader. However, we never hear him speak, with the voice being providedby James Earl Jones, rather than Prowse's native West Country accent.
Apparently he spoke the lines in his own voice while acting as Darth Vader, before the producers decided his Bristol/West Country accent wouldn't make the villain convincing, hence why he was dubbed with the voice of James Earl Jones. There's a special feature from A New Hope in which you can hear him saying the lines in his own voice: I think it's a scene from early in the film where the line is "I command you to tear this ship apart until you find those plans". It might even be on YouTube.