Dave Allen was originally considered for Jeff Randall (who was originally named Steve), but this was abandoned when he moved to The BBC.
Originally, Kenneth Cope was going to wear dark green suits as the ghostly Marty. This idea was soon scrapped.
Written-In Infirmity: Mike Pratt broke his leg prior to filming "The Ghost Talks", necessitating a re-write where Kenneth Cope carries the story and Pratt spending the episode in a hospital bed.
The Other Darrin: Felia was played by Jessica Hynes in the first season, and replaced by Pauline Quirke for her sole appearance in the second season. Considering that her characterisation seemed to totally change between the two seasons (Hynes's version was confident and self-assertive, but Quirke's version was a neurotic crybaby with OCD), it seems kind of strange that they bothered to keep her as the same character.