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. Many years later, at a tribute to director Sir Alfred Hitchcock, Bergman went off-script and presented the key to him, to his surprise and delight.
Sir Alfred Hitchcock got the shot where Ingrid Bergman is in the background and the coffee cup is in the foreground, with both in focus, by using a giant coffee cup placed farther away than it appears.
Sir Alfred Hitchcock claimed that the F.B.I. had him under surveillance for three months because this movie dealt with uranium. During production in early 1945, the use for uranium ore was not common knowledge due to secrecy. But, after the dropping of the atomic bomb on Hiroshima on August 6, 1945, and the release of details of the Manhattan Project, removed any doubts about its use.
Director Sir Alfred Hitchcock and screenwriter 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.
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 were slightly taller than Bergman, while Grant was actually about seven inches taller than Rains.
Alfred Hitchcock: (At around an hour and four minutes) 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.