Although the script instructed Leonard Nimoy to emote a fearful reaction upon his first sight of Big Balok, director Joseph Sargent suggested to Nimoy that he ignore what the script called for and instead simply react with the single word "Fascinating." The suggestion of this response helped refine the Spock character and provide him with a now-legendary catchphrase.
Sulu has transferred to the command division from the sciences division following his premiere in Where No Man Has Gone Before (1966). This the first production episode in which he occupies his familiar seat at the helm. The change was made because the producers realized there's no need for an astrophysicist in every episode, yet they had to have someone sitting at the helm.
Spock establishes that he is the product of a Vulcan father and a Terran mother. In Where No Man Has Gone Before (1966) his ancestry was not yet defined, as he gives the rather vague statement that, "one of my ancestors married a human female."
McCoy says "What am I, a doctor or a moon shuttle conductor?" which can be considered the first of the "doctor not a" quotes. In later days, the quote would have been phrased "I'm a doctor, not a moon shuttle conductor!"
Captain Balok and his vessel ''Fesarius'' are affiliated with the "First Federation." Capt. Kirk's ''Enterprise'' is still under the United Earth Space Probe Agency (UESPA, or "you-spa") which is mentioned in the next few episodes to be filmed. The name United Federation of Planets wouldn't be established until later on during the first season, although Starfleet would be used in the next broadcast episode, The Menagerie: Part I (1966).