While talking to Lewis about how she lost her magic and family, Florence Zimmerman's bare arm is shown, and a tattoo is visible implying that she was in a concentration camp.
The fictional "House with a Clock in Its Walls" was based on the real-life Cronin Mansion in author John Bellairs's hometown of Marshall, Michigan, which served as the basis for the town of New Zebedee used in his novels.
Screenplay writer Eric Kripke has stated multiple times on his Twitter page that he was a fan of the novel "The House With A Clock In Its Walls" while growing up. He has even stated that the novel was the original inspiration of the long running CW television show, Supernatural: Zur Hölle mit dem Bösen (2005), which he created. He has also said that he wrote in a few Supernatural references as a way of paying tribute, which includes the yellow-eyed demon Azazel.
The original novel was set in August 1948-April 1949, while the film gives a date of 1955, to make use of the post-World War II background for the characters.
Mrs. Zimmerman boasts that she once melted Salvador Dalí's watch "right off of his wrist." While the film doesn't elaborate, it's implied that this moment inspired Dali's painting "The Persistence of Memory," famous for its melting clocks.
Eli Roth: appears as Comrade Ivan in the black and white "Captain Midnight" scene that Lewis sees on the wall. He is also the voice-over on the school's PA.