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.
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.
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, whilst Grant was actually about 7 inches taller than Rains.
Alfred Hitchcock and Ingrid Bergman managed to get along famously, despite his infatuation with her. Hitchcock once told the story of how Bergman, attending one of the frequent dinner parties at his home, hysterically refused to leave his bedroom until he made love to her - an episode that almost surely never happened, but his obsession with the star was obvious enough to cause tension between him and his wife of many years, Alma Reville.
Alfred Hitchcock said he was inspired to do the kissing scene in part by the memory of a young couple he spotted from a train in France. The boy was urinating against a wall and the girl had hold of his arm, never letting go. "She'd look down at what he was doing, and then look around at the scenery, and down again to see how far he's got on," Hitchcock explained. "And that was what gave me the idea. She couldn't let go. Romance must not be interrupted, even by urinating."
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.
Whilst filming one shot, Cary Grant carped that he was supposed to open the door with his right hand, but he was holding his hat in that hand. "Have you considered the possibility of transferring the hat to the other hand?" Alfred Hitchcock replied.
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 (1946) 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.
The scene of Alicia drunkenly speeding along a South Florida road with Devlin as her passenger was shot in the studio with rear projection. The projected shots had a motorcycle cop gaining on them. As he gets closer to the car, he goes out of frame to the right, and the film cuts to him riding next to the car, this time in the studio. Hitchcock suggested to cinematographer Ted Tetzlaff that he shine light on the backs of Cary Grant's and Ingrid Bergman's necks as the projected motorcyclist moves off to their side. According to Alfred Hitchcock, Tetzlaff was irritated that the director thought of this instead of him and snapped, "Getting a bit technical, aren't you, Pop?"
To get around possible censor objections to kissing scenes that were too long and passionate, Alfred Hitchcock devised a scene where Cary Grant and Ingrid Bergman would neck and nibble at each other for a few minutes whilst they discussed food, moved about the apartment, and spoke on the phone.
Alfred Hitchcock and Ingrid Bergman's happy working relationship was enhanced by the opening of their previous picture in November 1945 during production of this movie. Spellbound (1945) received enthusiastic reviews and within weeks of its release was well on its way to earning eight times its cost.
Alfred Hitchcock was his usual unflappable self during production. Whilst in conference with Ted Tetzlaff on the set one day, a fire broke out. Hitchcock finished his sentence to Tetzlaff, turned to some stagehands and said quite coolly, "Will someone please put that fire out?" He then returned to his conversation.
Alma Reville may have had another reason for jealousy, according to biographer Donald Spoto. Alfred Hitchcock's longtime collaborator, script doctor, and adviser, she was often shunted aside during his successful writing partnership with Ben Hecht.
Alicia asks Dr. Anderson whether he's going to Leopoldina, a town in Minas Gerais, Brazil. According to Wikipedia, "Leopoldina was the site of a wave of immigration of Jews, mostly from Eastern Europe, in the 1920s and 1930s...."
Alfred Hitchcock: About an hour and four minutes 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.