"Nothing," Thornhill replies.
Supposedly this was a jab at David O. Selznick.
Lars Thorwald (Raymond Burr) in Rear Window was also a jab at David O Selznick.
Selznick brought Hitchcock to Hollywood from Britain in 1939 and they produced Rebecca, Spellbound, and Notorious. Selznick was a restless, controlling micromanager who obsessed over scripts and details.
Hitchcock had Burr styled to look like a dead ringer for Selznick giving him the producer’s signature curly gray hair and glasses. Hitchcock personally coached Burr to smoke and move with Selznick’s exact physical ticks and telephone habits. The resemblance was an open secret in Hollywood serving as Hitchcock’s lasting revenge for years of micromanagement and creative friction during their turbulent partnership.