The monster suit was deemed "not scary enough" on set, requiring serious post-production work to up the "scare factor" and keep things from becoming unintentionally comedic.
The title comes from the French and means "a madness shared by two," referring to the concept of a shared psychotic disorder.
Throughout the episode, Mulder's fingers are seen taped together. This is from the injury received in the previous episode, The Pine Bluff Variant (1998).
Until the show returned with Season 10, this was the last standalone episode to be filmed in Vancouver.