Boris Karloff as Dr. Henry Jekyll, inĀ Abbott and Costello Meet Dr. Jekyll and Mr. Hyde (1953)
Karloff only really played Dr. Jekyll in this film. Once the makeup transformation scenes were over, stuntman Eddie Parker did every scene as Mr. Hyde. This was even tipped off to audiences through publicity stills for the film, which showed both Karloff and Parker in makeup standing next to each other.
(via waterclap)