star wars - Why did three actors play Darth Vader?

In the original Star Wars movies (IV, V, and VI) three actors ended up playing Darth Vader. David Prowse was the actual physical body for the most part, James Earl Jones was used for the voice, and in Return of the Jedi when the helmet was taken off he was played by Sebastian Shaw (who was also one of the "ghosts" at the end of the movie originally but was edited out and replaced by Hayden Christensen).

So my question is, why was the decision made to have so many people play one character in the original movies? Did Prowse just not have the right voice or look right (except for his height)? Was Sebastian Shaw not tall enough?

Answer

David Prowse definitely did not have the right voice. He is from Bristol in the UK, and has a Bristolian accent, what would be described as in the UK as a 'west country accent'. To many ears this sounds a little parochial - most people would think this more the accent for a farmer than a galactic enforcer for the Emperor, however unfair that comparison might be. However David Prowse was a body-builder, had acting experience so he had the right look at 6 foot 6 inches tall. He was also from the UK where many of the scenes involving Vader were made.

James Earl Jones is to a degree the opposite - he has an amazing voice, and although a more accomplished actor than Prowse, does not have the right bodybuilder physical presence and is (only) 6 foot 2 inches tall.

Finally, why Sebastian Shaw and not Prowse for the final scene with Vader? They may have thought that Prowse, only in his late 40's, might look a little too young to be Luke's father, and the accent would have been too striking a difference to the 'in helmet' voice. Of course with the later casting of Hayden Christensen as the young Anakin, the age would have been about right.