Though this movie is officially based on the novel "A Shilling for Candles" by Elizabeth Mackintosh (writing under the name "Josephine Tey"), Sir Alfred Hitchcock and his writers only used about one-third of the novel and changed the identity of the murderer.
In America, the studios cut out the birthday party scene, which Sir Alfred Hitchcock said was absurd, as "that scene was the essence of the film."
(at around 18 mins) When the policeman tells the pig-cart driver that they need to commandeer his vehicle in the name of the law, he says, "This isn't a Black Maria." That slang term for a police prisoner transport vehicle originated in the U.S. It derived from an African-American Boston boarding house/tavern keeper, Maria Lea, who was so big and fierce that the local police sometimes asked her to help bring in unruly suspects. In addition to having an ethnic connotation like the synonymous Paddy Wagon, the term also traveled across the Atlantic and came into common use in England.
The inherent humor of policemen being forced to ride in the back of a pig wagon lies in the fact that in English criminal parlance police are referred to as "pigs".
Nova Pilbeam (Erica) was only 17 when she made the film, while Derrick De Marney (Robert), who falls in love with Erica was 31.
Alfred Hitchcock: Outside the courthouse holding a camera as Robert Tisdall (Derrick De Marney) escapes (at about 0:16:10).