For most of Star Trek, James Doohan ("Scotty") hides his right hand, which was missing the middle finger due to a WWII D-Day injury. While being questioned with his hand resting on the lie detector scanner, his fingers are hidden by being curled around the edge of the plate. During a close-up shot of the machine reacting to an intentional lie being told, a five-digit hand spread across the plate is seen - that of a stunt double. Doohan later wrote about it in his autobiography and said this was one of his favorite episodes.
This episode was made around the time that Leonard Nimoy earned an Emmy nomination for his portrayal of Mr Spock. William Shatner has admitted that this put his nose out of joint considerably, leading to tensions between the two actors, so this episode was a conscious effort on the part of the writers to sideline Nimoy and give Shatner center stage. (After the series finished, these animosities were put aside and the two remained lifelong friends up until a few years before Nimoy died in 2015.)
Robert Bloch originally wrote that Kirk, Scotty, and Bones having drinks which had colored layers. Their moods would change as they drank each layer. This idea was dropped as being too complicated of a special effect. Network censors were concerned that it would appear that they were using drugs. The producers argued against this but were unable to use the drinks anyway.
This is one of the very few episodes of the second season to feature music composed by Alexander Courage (mainly because of the feud between Courage and Gene Roddenberry, and his resulting withdrawal from the series).
According to Tanya Lemani, the makeup for Kara was initially much more elaborate. Lemani recalled, "They sent me to the makeup department because they wanted to do something extravagant with my look. The first day, they put feathers of different colours all over my face - on my eyelashes, my eyelids, my nose. Then they took me to the director, Joe Pevney, and he said, 'No. No. Less!' The makeup people kept trying to match his vision for four days, with less and less feathers and fewer colours each time, but Joe kept saying, 'No.' Finally, on the fifth day, I came in with no makeup and he said, 'That's it. That's what I want to see - her face." Lemani did her own belly dance choreography for the scene, but due to censorship concerns, had to cover her navel with a jewelled flower.