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."
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.
The failure of the original copyright holder to renew the copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS or DVD copy of the movie. Therefore, many of the versions available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies.
Alfred Hitchcock: Outside the courthouse holding a camera as Robert Tisdall (Derrick De Marney) escapes (at about 0:16:10).