The episode's plot was a clear indictment of the discrimination and prejudice which was still rampant in the late 1960s by showcasing its complete absurdity, especially in light of the assassination of Martin Luther King less than a year prior, and just a few years after the Watts Riots and the events later depicted in the films Ghosts of Mississippi (1996), Malcolm X (1992) and Mississippi Burning (1988). The white/black and black/white makeup was also a rather obvious allegory to the tension that existed between many whites and blacks, especially in the Southern United States. However, many critics charged that this underlying message was considered much too obvious and heavy-handed, overshadowing what was otherwise excellent acting by Frank Gorshin and the series regulars.
Every time there is a "red alert", the camera quickly and repeatedly zooms in and out of a shot of the one of the many flashing, red warning lights which indicate the red alert. This camera effect, no doubt used to generate some interest in the episode, was only used in this episode.
The characters of Bele (Frank Gorshin) and Lokai (Lou Antonio) both wear shirts which are not pullovers but instead zip up the back. This was because makeup application with the shirts on would have soiled the shirts, and pulling shirts over their heads after the makeup was applied would have disturbed the makeup. Therefore makeup had to be applied first, including below the neckline of the mock turtlenecks they will be wearing. Then the shirts could be put on gently and laid over the made-up neck, and then zipped snugly up the back.
During the filming of Frank Gorshin and Lou Antonio's run sequences, Gorshin and Antonio collided with one another when neither actor knew the other was striding down opposite ends of the corridor. The camera crew hadn't warned them that their scenes were being shot simultaneously.
The characters of Bele (Frank Gorshin) and Lokai (Lou Antonio) are depicted as wearing gloves all the time. This was not because it was a requirement of the script or character descriptions, but because the black and white makeup would have smudged and rubbed off every time their hands touched anything or any other character.
The screenplay was based on a story by Lee Cronin, the pseudonym of Gene L. Coon. He used a pseudonym because he had left Paramount and was under contract with Universal, so he was not supposed to be working for Paramount as well.
The original story concept did not depict the aliens with bi-colored skin. One was a devil with a tail and the other was an angel. Episode director Jud Taylor came up with the idea of bi-colored skin shortly before the episode began filming. His original suggestion was that they be half-black/half-white, one color from the waist up and the other from the waist down, but each wearing reversed color schemes. The central idea stuck but the colors were finally separated along the vertical axis rather than along the horizontal.
This episode features a close-up of the Enterprise model. Zoom shots from below and above the saucer section are used, representing some of the rare 'beauty shots' of the ship filmed during the series (episodes "Operation -- Annihilate!" and "Metamorphosis" have unique shots of the Enterprise as well). During the opening credits in the first scene, for example, the camera glides underneath the saucer to an extreme closeup of the saucer's phaser section and light. "That Which Survives" uses the same shot briefly when the Enterprise is shaking at warp.
The SciFi Channel, the DVD, and the remastered version added some new scenes that were not in the original and VHS version. After Kirk makes his first log entry at the beginning of this episode, he asks Chekov about estimated time to Ariannus, tells Uhura to contact them to tell them that decontamination is to begin on arrival, and asks Scotty if it will it present any danger. Then after the shuttle is bought to the hangar deck, there is a shot of the shuttlecraft docking with the Enterprise. Sulu then calls Kirk in the turbolift to inform him that hangar doors are closed. Finally, there is a shot of Kirk and Spock in the hallway before they meet with the guards.
The trivia item below may give away important plot points.
Frank Gorshin first came to public attention as a stand-up comedian and celebrity impressionist. James Cagney was an impression he was known for. In the old "Star Trek" blooper reels, there is a 'take' of Bele beaming down to Cheron. For completion of the scene, after leaping onto the transporter platform, Gorshin (as Bele) looks up and freezes. There is a call of "Cut" from the director, immediately upon which Gorshin turns to the camera, scrunches up his face and body like Jimmy Cagney and launches into, "Oooo, you dirty rat!"