The film staffers on Frankenstein were worried that Marilyn Harris, the 7-year-old who played the girl the monster throws in the lake, would be terrified by Boris Karloff's make-up. Instead she loved it, and ran up to him in delight as soon as she saw him. (Karloff did not throw her in a lake.) (Source)

Boris Karloff was so unknown he wasn't even credited at the beginning of Frankenstein. But the film made him so famous that in Bride of Frankenstein he's simply credited as "Karloff" because everyone knew who he was. Good career move, Boris. (Source)

Two famous actors, Bela Lugosi and John Carradine, both turned down the role of the monster because the monster doesn't talk. What actor wants a role where they just say, "Arrggggh! Grrr!" (Boris Karloff did, that's who.) (Source)