 A story about explicit immortality had been discussed for several years. However, the writers found it hard to make immortality seem scary. Frank Spotnitz, the show's executive producer, noted that after making the immortal character a photographer who was trying to catch Death so that he could die, the problem was solved. The episode was not the first to make a reference to immortality, however. The season three episode "Clyde Bruckman's Final Repose" featured Scully being told by the titular character that she would not die. This episode was the inception of a lesser-known story arc that was originally supposed to reveal that Scully was immortal. The sub-plot, popular with fans on the internet, was verified by Spotnitz. However, Spotnitz later admitted that this sub-plot was bookended by "Tithonus", a solution that Spotnitz later called "very satisfying".
