Tom Wright was eager to pursue the role of Tuvix when he first heard about it. While he had not previously watched Star Trek: Voyager (1995), he was acquainted personally with Tim Russ and Ethan Phillips, having often auditioned for the same roles as the former and having appeared in a play with the latter. When he originally read for the role, knowing little about Tuvok and Neelix, Wright instead based his portrayal on his knowledge and experience of Russ and Phillips.
This episode was originally written by freelance writers, with a much more comical tone and lots of slapstick elements. Staff writer Kenneth Biller said that it was almost like a farcical 1960s sitcom, and as a joke, he even came up with a funny theme song to go with it. However, he later decided that the premise, though somewhat hokey, deserved a more serious and philosophical approach, so he thoroughly re-wrote it to focus more on the dramatic angle.
Ethan Phillips was originally considered for the role of Tuvix, but director Cliff Bole decided against seriously pursuing the possibility, feeling Phillips as Neelix was "too identifiable."
The piece Harry is practicing on his clarinet is the first movement of the Sonata in E-flat Major, Op. 120, No. 2, by Johannes Brahms, a composer whose music was a plot device in Requiem for Methuselah (1969).
The science laboratory on Voyager is seen for the first time.