Actually, while I'm on the subject, that reminds me of a funny anecdote I heard on a making-of doco for The Hitch-Hiker's Guide to the Galaxy, the TV series. David Learner played the body of Marvin, and at one point during the series, David Prowse (aka the body of Darth Vader) joined the cast as Hotblack Desiato's body guard (the separation of the words is intentional, given the context).
Anyway, during rehearsals, David Learner found himself sitting on a couch next to David Prowse. Learner himself wasn't going to provide the voice of Marvin, and so asked David Prowse if he had any idea why George Lucas replaced Prowse's voice with James Earl Jones'.
Prowse's response? "Oi have no oideah."
Have you seen the size of human brains? They're hilarious!
-The Twelfth Doctor