Dawson Casting: Both leads were older than their characters (actually it's not clear how old Roger is supposed to be, given that Cary Grant was in his mid-50s at the time). Amusingly, legend has it that 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.
Reportedly, the less-than-spectacular box office results for Vertigo caused a falling out between Hitchcock and Stewart, hence no North by Northwest for the latter.
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!
Word of Gay: Martin Landau stated in interviews that he portrayed Vandamm's henchman Leonard as a closeted homosexual who was secretly jealous of Eve Kendall's relationship with his employer. In the scene where he revealed to Vandamm that Eve was secretly working for the Feds, he commented, "Call it my woman's intuition, if you will..."