Dawson Casting: Both leads were older than their characters. Amusingly, Hitchcock denied Jimmy Stewart the part of Thornhill because he thought he was too old for it - so instead he got Cary Grant, who was even older than Stewart.
Throw It In: An unintentional one. When Eve shoots Roger in the restaurant with the blanks, a child actor covers his ears before the gunshot happens. The take was still used, since aside from this, it was the best take.
What Could Have Been: While the film was still in the early scripting stage, a scene taking place at an assembly line in Detroit was envisioned, where the camera would follow two men as they talked while a car was built behind them piece by piece...and at the end, one of the characters would open a door of the now-finished car and a corpse would fall out!