Sir Alec Guinness was spotted in costume while walking home through the French countryside. A young boy ran up to him, yelling "Mon père! Mon père!" ("My father! My father!") Guinness did not speak French, so he could not correct his mistake, but was touched that the boy apparently immediately bonded to him on the assumption that he was a priest. Soon after this movie was released, Guinness converted to Catholicism.
At the auction the Texan bids $50,000 for the chess set, which is immediately translated by the auctioneer to £17,857 two shillings and tenpence. This calculates correctly to $2.80 to the pound sterling, the standard and fixed exchange rate from 1949 until devaluation in 1967.
St. Augustine, referred to in this movie, is not the famous Saint Augustine of Hippo, a bishop and theologian who lived in 4-5 century A.D. in the Roman province of Numidia, Africa, and of whom many viewers may have heard. This movie refers to St. Augustine of Canterbury, no less famous in England, who is known as "Apostle of the English". He was an abbot in Rome, and was commissioned by Pope Gregory the Great in 596 A.D. to lead a perilous mission to England with his monks to baptize the Anglo-Saxon tribes where the Church had lost its ground many years before that followed the withdrawal of Roman legions from Britain. After the first failed attempt to reach the channel, St. Augustine returned to Rome, and on the second attempt he and his monks finally reached England in 597 A.D. St. Augustine established his mission in Canterbury, Kent and became the first Bishop there, and the head of the local church.
The parable about the woman and the onion Fr. Brown relates to Flambeau is from "The Brothers Karamazov" by Fyodor Dosteovsky (1821-1881).
Director Robert Hamer struggled to cast the character of Father Brown. He originally wanted Alastair Sim but Sim was busy filming two films back to back, 'The Belles of St Trinians' (1954) and 'An Inspector Calls' (1954). Hamer eventually asked his friend Sir Alec Guinness, who did it as a favor to help Hamer despite feeling he was too young for it.