Nigel Green died shortly after production from an overdose of sleeping pills; it was ruled an accident but is believed by some to have been a suicide, as Green was said to have been greatly depressed during filming. It had already been decided that his dialog should be replaced by that of another actor in the finished film, Graham Crowden.
Alastair Sim cast himself in the role as the Bishop, according to the commentary on the DVD. He called up his friend Peter O'Toole and told him, out of the blue, of course he'd help him by taking the part. No one was about to turn down the offer of such a great character actor, no matter how eccentric.
In the commentary on the DVD Peter O'Toole says that he did the stunt riding the horse to the hunt himself, because he owned a racehorse, Eric the Red, who'd been a good boy, won a few races and was now retired, and he, O'Toole, had always had this dream that he wanted to ride a real thoroughbred racehorse at a flat-out gallop.
Alastair Sim modeled his characterization of Bishop Lampton on the Primate of All England (chief religious figure in the Church of England), Michael Ramsey (subsequently Baron Ramsay of Canterbury). Portraits of Ramsay in the National Gallery bear a likeness to the physical aspects of Sim's character.
The film's writer, Peter Barnes, went to school just outside Bristol, in Stroud. Also just outside of Bristol is a mental hospital, in the village of Barrow Gurney; "Barrow Gurney" has long been associated in local patois with mental illness. Naming the central family in this play "Gurney" seems more than mere coincidence.
The filming of the wedding required several takes, because the actors found it impossible to keep a straight face during the scene. This was partly due to the hilarity of the scene itself, partly to Alastair Sim who did his best to make the other actors laugh when he himself was not being filmed. Carolyn Seymour had the advantage that her face was hidden under the veil.
Jack's maiden speech before the House of Lords as the Earl of Gurney concludes with a near-perfect quotation of three passages from the King James Bible: Deuteronomy 20:3, Deuteronomy 20:4 and Ezekiel 7:9. ("Approach this day unto battle against your enemies. Let not your hearts faint, fear not and do not tremble, neither be ye terrified because of them. For the Lord your God is He that goeth with you to fight for you against your enemies to save you. And mine eyes shall not spare, neither will I have pity. I will recompense them according to their ways and their abominations that are in the midst of them. And they shall know that I am the lord that smiteth.")