Awesome, Dear Boy: Teresa Wright loved the plot outline Alfred Hitchcock gave her so much that she signed on without reading the script.
Magnum Opus Dissonance: Other Hitchcock films are better remembered today—Psycho, North By Northwest, Vertigo, Rear Window—but, as noted on the main page, Hitchcock considered this his favorite of his own works. It should be noted during Hitchcock's lifetime, Shadow of a Doubt was considered by critics to be his best film too, the later films come in the 50s were seen as glossier productions at the time. Teresa Wright also said this was her favourite film.
Reality Subtext: Alfred Hitchcock put a lot of personal references into the film. His mother's name was Emma, like Charles's sister. Young Charlie's father is called Joseph, which was his middle name. He had a bicycle accident when he was young, like Charles. Ann is reading Ivanhoe near the beginning, which was his favourite book as a child. Herbie has an obsession with murder, just like the director. And finally, the car Charlie drives is the same as the one Alma Reville loved driving.
Unintentional Period Piece: Pa Newton says he's never worn a wristwatch before and thinks it will make him look fancy. Otherwise averted, as the movie never mentions the second World War, aside from a brief headline in a newspaper Charlie reads "Tojo Speaks For..." - Tojo Hideki was the Prime Minister of Japan for most of the war.