The Servant
(1963)

Barrett (Dick Bogarde) cons Tony (James Fox) into giving him his job back.  Barrett and Tony become friends, confidantes, and soon Tony comes to think of Barrett, his servant, as his equal.  Barrett uses his position of trust to manipulate Tony into total dependence on him, and by the end of the movie the two have essentially reversed their roles -- with Barrett acting as the master and Tony, the dim-witted playboy, as his servant.

Thanks Greg B!