In his book "Star Trek Memories", William Shatner identified this as his favourite episode, because his father died during filming and Leonard Nimoy's delivery of the mind meld lines made him laugh. He thought it was "exciting, thought-provoking and intelligent, it contained all of the ingredients that made up our very best Star Treks."
Janos Prohaska, the creator of the Horta costume, actually wore it into Gene L. Coon's office, as if to say "Look what I designed". Coon said "That's great! What is it?", and Prohaska said "I don't know. It can be whatever you want." Coon replied "I'll write a script around it", and he wrote this episode in four days so the costume could be used.
When William Shatner, on the set, got the call from his mother informing him about his father's death, the crew was ready to shut down production, but he insisted on continuing. During the rest of the day, Shatner took comfort in Leonard Nimoy, and cinematographer Gerald Perry Finnerman, whose father had died on a movie set less than seven years before.
Arthur C. Clarke once remarked, in 1995, that of the Original Series, the only episode he could recall was this one, stating that "It impressed me because it presented the idea, unusual in science fiction then and now, that something weird, and even dangerous, need not be malevolent. That is a lesson that many of today's politicians have yet to learn."
Leonard Nimoy identified this episode's closing banter between Spock and Kirk as one of his favourite scenes to perform. He noted, "It was a wonderful moment which defined the relationship and defined the whole Spock character's existence and his attitude about himself."