This script deals with immortality. The entire cast all lived exceptionally long lives. Kevin McCarthy lived to be 96, Estelle Winwood was 101 when she passed away, Edgar Stehli passed away shortly after turning 89, and Dodie Heath died at the age of 96 in June of 2023
The revolver Laurette uses to shoot Walter is a Colt 1862 (with an after-market cartridge conversion kit); an apt gun, for a man who fought in the Civil War, since it was first sold during the Civil War, and was a common side-arm, for both Union and Confederate soldiers.
Coincidentally, the character portrayed by Kevin McCarthy in this episode mentions the date of September 11th. McCarthy died on September 11th, 2010.
The interior living room & front door entrance set from the first act is the living room of "George" in The Time Machine (1960). The original wood paneling from that film has been painted white, but is otherwise obviously the same set, albeit without the Victoriana dressings. The same set also appears in other first season episodes of The Twilight Zone.
This episode may have inspired fellow "Twilight Zone" writer Jerome Bixby (It's a Good Life (1961)) to write his novel "The Man From Earth", also a story about a history professor who reveals to his friends that he is immortal - though the tone, the age of the lead character, the mechanism of immortality, and the plot of the stories all differ considerably. Bixby also visited the "immortal man" theme in Requiem for Methuselah (1969).