The hands performing the brain operation are actually that of a real neurosurgeon at Cedars of Lebanon Hospital in Los Angeles, hired by the producers to make the operation look authentic.
Released by United Artists as part of a double feature with The Quatermass Xperiment (1955). The parents of Stewart Cohen attempted to sue the Lake Theater and distributors United Artists for negligence after their nine-year-old son died of a ruptured artery in the cinema lobby at one screening. Cohen entered the Guinness Book of Records as the only known case of someone literally dying of fright at a horror film.
Bela Lugosi was reportedly very unhappy with his lack of dialogue and reportedly pestered director Reginald Le Borg to give him some lines. Although Le Borg did eventually shoot some extra dialogue scenes with the actor, they were never used. (Chaney doesn't speak either, but apparently he didn't care as much.)
When Peter Lorre's salary demands were deemed too excessive for the limited budget, he was replaced by Akim Tamiroff. Although Tamiroff makes no attempt to imitate Lorre's voice, he does mimic his facial expressions, mannerisms and vocal cadence to a considerable extent.