Peter Lorre and Jack Nicholson were fond of ad-libbing their lines, much to the annoyance of Boris Karloff, who was working from the script.
Jack Nicholson always gave high praise to everyone he worked with on the set, except one - the Raven. He said the Raven pooped on everyone, but especially liked to poop on him. He later said: "I would look down when the Raven flew off my shoulder, and it would be covered in poop. I hated that bird."
In casting his spells, Dr. Adolphus Bedlo (Peter Lorre) used several Latin phrases: Veni vidi vici: I came, I saw, I conquered. De mortuis nil nisi bonum: Do not speak ill of the dead. Cave canem: Beware of the dog. Si vis pacem para bellum: If you want peace, prepare for war. Ceterum censio Carthaginem esse delendam: Furthermore, I believe that Carthage must be destroyed.
According to Roger Corman, Peter Lorre's improvisations confused Vincent Price and Boris Karloff, but Price adapted to this well, while Karloff struggled.
Peter Lorre ad-libbed several famous lines in the movie, including: "How the hell should I know?" after Price asks if he will see Lenore again; "Hard place to keep clean, huh?" in the family crypt, while retrieving dead man's hair; and "Where else?" in the family chapel, when Price says he keeps Lenore there.