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?