Visual effects supervisor Laurie Kallsen-George created the glowing red eyes, digitizing and blending various eyeball images, including shots of her family's dog.
Filming had to be moved to a soundstage after all the incessant rain on location. The famous sleeping bags/ "Joy to the World" exchange between Mulder and Scully was shot indoors, much to David Duchovny and Gillian Anderson's relief.
Gillian Anderson deliberately sang "Joy to the World" slightly out of key, in keeping with her character who "can't carry a tune." She says she can sing it a lot better in real life.
Singing "Joy to the World (Jeremiah was a Bullfrog)" to Mulder is one of Gillian Anderson's favorite moments in the entire series.
Most episodes are shot in about 8 days. Continual rain meant that this episode faced numerous delays and came in after 19 days.