William Shatner had a severe case of flu during filming of this episode. At one point he had to lift Sandra Smith in his arms, carry her to a couch and put her on it: during the first take he got as far as the couch and dropped her. Fortunately it was well padded, and Smith bounced several times; according to Joanie Winston, who was visiting the set, Shatner looked down at Smith and said, "You know I love you, baby, but you've got to lose about six inches off that ass."
In Roman mythology, Janus was a 2-faced head representing opposite aspects in nature such as chaotic nature and controlled civilization. "Lester" means in Old English: locality. Hence, Janice Lester. A split personality from nearby or within.
"Turnabout Intruder" which is the final episode of the original series, takes place in 2269, 4 years before "Star Trek: The Motion Picture" which took place in 2273. "Turnabout Intruder" was broadcast a decade before the 1979 feature film.
According to 'Harry Landers', he was very fatigued during this episode because he had just has his upper right lung removed due to an infection. He wanted to turn the role down but did it as a favor to producer Fred Freiberger.