While director Robert Day and the make-up man were discussing how to achieve Boris Karloff's metamorphosis without undue complication or expense, the actor volunteered that he could achieve the effect by taking out his dentures, which he had also done when he played Frankenstein's monster.
One scene shows an evidence box marked Constance Kent, the child murderer who was found guilty in 1865 of killing Francis Kent at Road Hill House in Wiltshire. Spared the death sentence, she was released in 1885 and emigrated to Australia, where she died in 1944 aged 100 years old.
Producer John Croydon first met Boris Karloff on the set of The Ghoul (1933) 25 years earlier as a tea boy.
Elizabeth Allan and Diane Aubrey play mother and daughter; it is the former's final theatrical film and the latter's first.
The asylum kitchen maid sings the popular Victorian music hall song "The Rat Catcher's Daughter", written by the Rev. Edward Bradley and Sam Cowell in the early 1860s. In the song, the unnamed daughter drowns herself in the River Thames after being rejected by a man nicknamed "Lilly White Sand" (who sells sand dug out from the river's bank). In the last verse the remorseful Lilly White cuts his own throat with a shard from a broken pane of glass. Although not sung in the film, contemporary London audiences would recognized this musical nod to the previous scene where Rankin/Tennant has slashed the guard's neck in the same way.