Claude Rains has a unique voice with an excellent script
Despite his visible absence in virtually all of the movie, Claude Rains' vocal technique is quite remarkable and unique. His aristocratic bearing is overshadowed by his defined individuality. In this role he dominated the screen through his antics and his license to exaggerate his character's insanity. Quite different from the debonaire parts he played with Bette Davis later on.
I also love the way he portrayed the arrogance that his medicine had altered his personality to in a Jekyll Hyde manner. He didn't just play the typical mad scientist. There's so much more humanity and characterization here. He actually delivered much more of a characterization in his part than the book imparted - which says much about how good the script writing was for this movie. And Claude Rains was as perfect in his part as Peter Lorre was in "M".