John Barrymore, who plays both Dr. Henry Jekyll and Mr. Edward Hyde in this movie, was such a gifted physical actor that the initial part of his transformation has no makeup. It’s him contorting his body and appearance all on his own.
This adaption of Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson was written by Clara Beranger (who was one of the original faculty members at USC School of Cinematic Arts) and was directed by John S. Robertson, who The Byrds wrote the song “Old John Robertson” about.
Henry Jekyll (Barrymore) is led to believe that all men have two sides at constant war for their souls: a good and evil brain, basically. A potion that he creates allows him to access that evil side of his being, unleashing Edward Hyde. Yet by the end of the film, the potion is no longer needed and the transformation comes whenever Jekyll becomes upset.