Factual error: When Cary Grant is attacked by the crop duster, he is supposed to be between Chicago and Indianapolis. Unfortunately, northwest Indiana looks nothing like this. The scene in the movie has miles and miles of treeless landscape. Even in the flattest farmlands of Indiana, there are many visible groves of trees.
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: 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."
Question: Several times in the movie one character is able to ascertain in which hotel room another character is staying simply by asking the front desk for the room number. Was this realistic at the time the movie was made? Today, a hotel would never divulge a guest's room number to a stranger, since such information could potentially be used by burglars and/or predators to gain access to hotel rooms. Was security really that lax in the 1950s?
Question: Why do the kidnappers take Thornhill to the Townsend home and pretend to be Lester Townsend and members of his household? They could have taken him to some obscure place instead, at less risk of being found out.