When Hitchcock is being driven home by Janet Leigh, when she questions his eating of the candy corn (which is not as posh and refined as his usual taste), he says "needs must when the devil drives". This is an old British phrase used in several William Shakespeare plays that means when one is in a desperate situation, one must do things they don't normally do. This line and scene are meant to imply that he is agitated and his mental state is not what it normally is.
Helen Mirren, who plays Hitchcock's wife Alma Reville, had also met the real Alfred Hitchcock when he approached her for a part as a murder victim in his penultimate film, Frenzy (1972). Mirren turned down the role, a decision she later regretted.
Although many reviewers criticized the film for inventing an intimate relationship between Alma Reville and Whitfield Cook, the facts are documented by more than one Hitchcock scholar, as exemplified by Patrick McGilligan in his biography of Alfred Hitchcock. That writer accessed Cook's private diaries available in the screenwriter/playwright/author's papers donated to the Howard Gotlieb Archival Research Center, Boston, Massachusetts.
The trivia items below may give away important plot points.
In the end scene, a crow lands on Hitch's shoulder, indicating that his next project will be The Birds (1963). That film, and Alfred Hitchcock's obsessive relationship with leading lady Tippi Hedren were the basis for a television-film about Hitchcock, The Girl (2012), broadcast the same year as this movie was released in cinema.
Over the course of the movie, whether on purpose or not, Hitch is subtly starting to take over habits from Norman Bates. For example, he accepts candy corn from Janet (which Norman habitually eats in Psycho (1960)), and at one point he is listening to Ludwig van Beethoven's Third Symphony ('Eroica') in his house (which Norman had on his record player). Although not addressed in the movie, Alfred Hitchcock also had a difficult relation with his mother, just as Norman did.