When Peter Lorre arrived in Great Britain, his first meeting with a British director was with Sir Alfred Hitchcock. By smiling and laughing as Hitchcock talked, the director was unaware that Lorre, a Hungarian, had a limited command of the English language. Hitchcock subsequently decided to cast Lorre in this movie, and the young actor learned much of his part phonetically.
The dentist scene originally was intended to take place in a barber shop. However, Sir Alfred Hitchcock saw I Am a Fugitive from a Chain Gang (1932), in which there is a scene exactly like it, so he changed it to a dentist's office.
Producer C.M. Woolf hated this movie and only allowed it to be released as the bottom half of a double bill. Nevertheless, it won rave reviews.
The crucial cantata for the Albert Hall sequence was composed specifically for this movie by Arthur Benjamin, and the same piece was used again in The Man Who Knew Too Much (1956). When Sir Alfred Hitchcock remade this movie, he offered composer Bernard Herrmann the opportunity to compose a new work for the scene, but Herrmann chose not to, citing an appreciation of Benjamin's original cantata.
Alfred Hitchcock: (At around 36 mins) Just after the dentist's office scene on the lamp post corner, entering stage right in a raincoat as a bus passes.