This is the only episode to feature all three prominent female characters in TOS: Lt. Uhura, Nurse Chapel, and Yeoman Rand.
Lt. (jg) Joe Tormolen is the only Lieutenant Junior Grade shown in series history, and this is the only time in TOS the rank is mentioned. Every subsequent Trek series, amusingly enough, has at least one Lt. (jg) regular character (except for Enterprise, whose Cast Calculus very deliberately apes that of TOS).
Listed Trivia:
The Cast Showoff: George Takei didn't know how to fence prior to being cast, though he said he could learn in order to better his chances at landing the role; when granted the role of Sulu he went out that weekend to do just that. In this episode he finally got to put his skills to practice after suggesting that he fence instead of run around with a katana playing samurai, as the writers initially intended.
Edited for Syndication: During the syndication run, the following scenes were typically cut from broadcast
An opening shot of the rec room showing crew members playing three dimensional chess.
Conversations on the bridge about the planet's break-up, later followed by a slightly longer exchange between Kirk and Uhura as to communication status.
A longer scene of Tormolen's operation.
An extended scene of Riley moving through the ship's corridors and blowing on the sickbay doors to have them open.
Sulu pricking his thumb on the edge of his foil.
Extended shots of Scott working in the Jefferies tube.
Longer establishing shots of sickbay, including McCoy sitting and Spock entering to speak to Chapel.
A scene where Kirk walks from the bridge down to engineering to check on Scott's status. In the syndicated version, the scene cuts directly to Kirk already at the door to engineering.
Spock walking towards the briefing room, about to have his emotional breakdown.
One-Take Wonder: The scene in which Spock cries was shot in one take.
Science Marches On: McCoy works out the 'disease' is actually water, which has changed to a complex chain of molecules that acts like alcohol in the bloodstream. This is a reference to polywater, a syrup-like form of water created from massive compression of normal water that has a lower freezing temperature, which allowed it to stay fluid despite the environmental controls on the space station the substance was on being set to freeze everyone onboard to death. Thing is, while polywater was debated as a viable substance in the 1960s it has since proven to be bogus (forcing an awkward change of the term used to describe the substance from a water to a virus in the Next Gen episode The Naked Now.
Throw It In!: Urban legend has attributed two classic moments to improvisation by the actors, but both appear in the final shooting script:
Uhura's response to Sulu calling her a 'fair maiden' - "Sorry, neither" - was said to be an ad-lib by Nichelle Nichols. Some sources say she ad-libbed it during rehearsals, which would have allowed it to be retroscripted into subsequent revisions (the show was notorious for having scripts rewritten as they were being shot - which is in fact the reason for William Shatner's frequent...pauses).
The scene in which Spock cries also appears in the original script, though legend attributes the whole thing to Leonard Nimoy coming up with it on the fly and improvising it in a single take. Nimoy did come up with many aspects of his character on the fly (the Vulcan Nerve Pinch, the mind meld, and the Vulcan salute were all improvised by him on-set), but his innovation here was primarily execution. The scene as scripted was far more comical and exaggerated, and he suggested that it be played quietly, with Spock alone and trying to regain control.
Wag the Director: The original script would have Spock crying while walking through the corridors, but the focus was on his odd reaction to the antics of everyone around him rather than the turmoil he was facing. Nimoy, having crafted much of the social and mental attributes of Vulcans, was opposed to how it played out and demanded a new scene that was more somber. This became a more somber, single shot scene where he walks into a conference room and has a mental breakdown, which Nimoy had to get right on the first take due to the budget constraints.
In the original script, it was Lieutenant John Farrell, who sabotaged main engineering, but in later rewrites, the character was replaced by Kevin Riley.
This was originally a two-part story that would lead into "Tomorrow Is Yesterday". This is why the former ends with an out-of-the-blue time-travel incident that leads nowhere and the latter begins with the Enterprise already back in time due to (in the aired episode) an unrelated time-travel incident.
According to writer John D.F. Black, at the time both Riley and Tormolen were under consideration to become regulars.
This is the first episode in which Eddie Paskey has dialogue. Originally, his character was to have said more lines. Not having any other acting job than being an extra, he was so nervous that a few of his lines had to be said by Spock instead.
, a syrup-like form of water created from massive compression of normal water that has a lower freezing temperature, which allowed it to stay fluid despite the environmental controls on the space station the substance was on being set to freeze everyone onboard to death. Thing is, while polywater was debated as a viable substance in the 1960s it has since proven to be bogus (forcing an awkward change of the term used to describe the substance from a water to a virus in the Next Gen episode The Naked Now.