Barbara Anderson developed a fever blister/cold sore on her lip during filming. Besides using makeup to partially disguise it, she was often filmed with part of her lower face in shadow.
In the original draft, the character whose parents had been murdered by Kodos was named Lt. Robert Daiken. When Bruce Hyde was cast in the role, the staff realized that the same actor had already played a character named Kevin Riley in the episode The Naked Time (1966), so the character was renamed accordingly for the sake of consistency.
The script began with a view of the city on Planet Q, with a poster advertising the Karidian Company of Players superimposed over it, then dissolving into the performance of Macbeth. Director Gerd Oswald decided this lead-up was unnecessary and chose a more gripping opening of Macbeth's dagger shown close up.
The dreamlike song sung by Uhura is "Beyond Antares." Nichelle Nichols got to interact with the Vulcan harp again in Elaan of Troyius (1968), but that scene was cut. In The Changeling (1967), she sings a portion of that song again, without an instrument, before Nomad interrupts her.
A scripted line, cut from the episode, established that Kirk was a midshipman, fresh out of the Academy when he was stationed on Tarsus IV and witnessed the massacre. Since it happened twenty years before the events of this episode, this would have indicated that Kirk is somewhat older than what was later established in The Deadly Years (1967). (It should be noted, however, that, while Academy cadets enter as midshipmen, they graduate as ensigns.)