In the original script, Uhura was to amuse the crew by performing as a trained mimic, imitating Spock and other officers. This was changed to her singing a song about Spock, followed by a spoof of the 18th-century Scottish song "Charlie is my Darling", the best-known version of which is attributed to Robert Burns, in order to highlight Nichelle Nichols' singing talent.
True to his training as a Method actor, Robert Walker Jr. chose to remain in his dressing room and not interact with any members of the cast as this would help his characterization of a strange, aloof person.
During the lounge scene, where Uhura sings a song about Charlie, Spock is seen smiling as he accompanies her on a harp-like instrument. This is one of the few times in the series that Spock smiles, while not under the influence of a substance or someone's mind control powers.
Some of the things now considered everyday items in Star Trek are missing during the early episodes. In this one, the Enterprise has a cook who prepares meals for the crew. The yeoman also talks of searching through "ship's stores." The use of replicators to create food and other materials had not yet been conceived of but would become commonplace by Tomorrow Is Yesterday (1967).
Robert Walker Jr. was 26 when he played the 17-year-old Charlie Evans. Pat McNulty (Tina Lawton) was 23. Grace Lee Whitney was 36.
Gene Roddenberry: voice of the galley chief who says to Kirk, "Sir, I put meat loaf in the ovens. There's turkeys in there now... real turkeys!" This was his only speaking role in "Star Trek." During the second season, his disembodied hand appears in a few scenes of Who Mourns for Adonais? (1967).