Trivia: Cary Grant was born on January 18th, 1904, and the actress playing his mother, Jessie Royce Landis, was born on November 25th, 1896, making her just seven years older than Cary. According to commentary on the DVD, it was thought that casting Landis as Cary Grant's mother would make Cary look young enough to be a believable love interest for Eva Marie Saint.
Trivia: After Eva Marie Saint snatches the sculpture containing the microfilm from James Mason, Cary Grant remarks, "I see you got the pumpkin." This was evidently a reference to the Alger Hiss case, in which Whitaker Chambers testified that Hiss had given him government secrets that had been hidden in a scooped out pumpkin in a pumpkin patch in Hiss' back yard.
Trivia: In the auction gallery, Cary Grant says to Eva Marie Saint, whom he sees with the spies for the first time, "The three of you together: here's a picture only Charles Addams could draw." This referred to a New Yorker cartoonist who specialized in macabre and grotesque characters. The reference would probably have been lost to Italian viewers, but the dubber did his homework, and the translation was, "The three of you together: just like something out of Dracula and Frankenstein."
Trivia: Thornhill is looking up at the Mount Rushmore sculptures, using the fixed, standing binoculars for tourists, and he exclaims: "I don't like the way Teddy Roosevelt is looking at me." The thing is, Theodore Roosevelt, the third from left to right, is the only one of the four heads to be looking as much away from him as could possibly be. I wonder why the screenwriter, Ernest Lehman, made this particular choice? Perhaps he was having a joke on the perceived ignorance of the contemporary cinema-going public at the time. Either that, or it was just sloppy writing. My guess is he just asked some underling: "Who are the four presidents represented on Mount Rushmore?", and then picked one at random.