Screenwriter Jay Presson Allen wrote in Mark's hobby of studying animal behavior because that was her hobby and tangentially fit with Mark's later inclination to psychoanalyze Marnie.
The 6 & 1/2 carat blue-white flawless diamond ring that Mark buys for Marnie cost $42,000, or approximately $285,000 in 2008 currency, when factored for inflation. In the week that Mark had returned with Marnie up until their marriage, he spends approximately $70,000, including the ring, or approximately $475,000 when factored for inflation.
The company created for copyright purposes for the film, "Geoffrey Stanley," was named after Hitchcock's pet dogs.
To film real horses riding without having to work outdoors, Alfred Hitchcock came up with the idea of running the horses on a gigantic treadmill. Crew members objected to the idea because it was considered highly unsafe and because they simply didn't think it would work. Still, Hitchcock wanted to at least try it, and when they did, it worked without a problem. Originally, a harness was attached to Tippi Hedren during these shots for safety reasons, but it was removed when it was found to impede shooting.
When Louise Latham came onto the set in her "young" makeup to film the film's climactic flashback, she looked so different that the cameraman began to ask around to find out who the new actress was.
When Alfred Hitchcock's discussion with Grace Kelly (to appear as the title character) became public, the residents of Monaco expressed their disapproval, and Kelly withdrew. In a further complication, since Kelly had not fulfilled her MGM contract when she married Prince Rainier of Monaco in 1956, she could not appear in any film other than an MGM film until she fulfilled the terms of her MGM contract.
Alfred Hitchcock:
Five minutes into the film, in the hotel corridor as Marnie walks by.