"Lux Radio Theater" broadcast a 60 minute radio adaptation of the movie on January 26, 1948 with Ingrid Bergman reprising her film role.

"The Screen Guild Theater" broadcast a 30 minute radio adaptation of the movie on January 6, 1949 with Ingrid Bergman reprising her film role.

Leopoldine Konstantin played the mother of Claude Rains but in real life she was only 4 years older than Rains.

Leopoldine Konstantin's performance as Madame Sebastian was her only role in an American film.

David O. Selznick sold the rights to RKO Pictures in order to finance part of Duel in the Sun, which was over-budget and behind schedule.

Lester Dorr was cast as a motorcycle policeman but was cut from the released print.

Ethel Barrymore was offered the part of Madame Sebastian but turned it down.

Alfred Hitchcock and Ben Hecht consulted Nobel Prize winner Dr. Robert Millikan on how to make an atomic bomb. He refused to answer, but confirmed that the principal ingredient, uranium, could fit in a wine bottle.

Alfred Hitchcock claimed that the FBI had him under surveillance for three months because the film dealt with uranium.

Alfred Hitchcock: [stairs] Final scene takes place on stairs.

Alfred Hitchcock: About an hour in, at the party in Alexander Sebastian's mansion, Hitchcock gets a glass of champagne from the bartender and quickly turns to the left and walks off screen.

Claude Rains was made to stand on a box for several of his scenes with Ingrid Bergman (not, however, in the honeymoon return scene). This gives the strange effect that Rains and Cary Grant are both slightly taller than Bergman, while Grant was actually about 7 inches taller than Rains.

After filming had ended, Cary Grant kept the famous UNICA key. A few years later he gave the key to his great friend and co-star Ingrid Bergman, saying that the key had given him luck and hoped it would do the same for her. Decades later at a tribute to their director Alfred Hitchcock, Bergman went off-script and presented the key to him, to his surprise and delight.

Both Cary Grant and Ingrid Bergman found the famous kissing scene quite problematic, according to Alfred Hitchcock, because of the complicated blocking that needed to be remembered in the several long takes that it took to shoot it.

Producer David O. Selznick originally wanted Vivien Leigh to play Alicia.

RKO bought David O. Selznick's package, consisting of Ben Hecht, Alfred Hitchcock, Cary Grant and Ingrid Bergman for $800,000 and fifty percent of the profits.

The legendary on-again, off-again kiss between Cary Grant and Ingrid Bergman was designed to skirt the Hayes Code that restricted kisses to no more than three seconds each.

The set used in this film for the interior of the house of Alex Sebastian (Claude Rains) can also be seen in the RKO production The Locket as the house of Mrs. Willis (Katherine Emery). This is especially noticeable in scenes filmed in the part of the set representing the second floor corridor.