According to Wikipedia, Alfred Hitchcock mandated that movie theaters could not allow late viewers to come into Psycho after it began.
The most controversial move was Hitchcock's "no late admission" policy for the film, which was unusual for the time. It was not entirely original as Clouzot had done the same in France for Diabolique. Hitchcock thought that if people entered the theater late and never saw the star actress Janet Leigh, they would feel cheated. At first theater owners opposed the idea, claiming that they would lose business. However, after the first day, the owners enjoyed long lines of people waiting to see the film.
How did Hitchcock enforce this restriction? I find it hard to imagine in our modern day that Steven Spielberg, for instance, could tell every movie theater in America how to run their operations.