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."
In a book about Star Trek, it was reported that after William Shatner returned from the funeral, to put everyone at ease, as he was trying to do his lines following Mr. Spock's mind meld with the Horta and his cry of "AHH! PAIN! PAIN! PAIN!" Leonard Nimoy just spoke the words so Shatner told him to do it again with feeling. When "Spock" again said "AHH! PAIN! PAIN! PAIN! " Shatner yelled out, "WILL SOMEBODY PLEASE GET THIS VULCAN AN ASPIRIN!"
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."
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.
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."
William Shatner was in Florida for his father's funeral while nearly all of Spock's "mind meld" scene with the Horta was shot. His screen double is shown from behind in several of the shots and all of Kirk's "reaction" shots were made after he returned.
Gene L. Coon's original script featured a different material as the base of the Horta, but researcher Kellam de Forest changed it to silicon, as the original choice seemed to be even theoretically impossible.
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.
Gene Roddenberry was impressed with the way this episode explains the behaviour of a Star Trek "monster," citing the instalment as "a classic example of doing this right" as well as "one of our most popular episodes." He went on to say, "The Horta suddenly became understandable [....] It wasn't just a monster-it was someone. And the audience could put themselves in the place of the Horta... identify... feel! That's what drama is all about. And that's it's importance, too... if you can learn to feel for a Horta, you may also be learning to understand and feel for other Humans of different colours, ways, and beliefs."
Bantam Books published a series of novelizations called "foto-novels," which took photographic stills from actual episodes and arranged word balloons and text over them to create a comic book-formatted story. The ninth installment was an adaptation of this episode. In it, Leslie is depicted as thinking to himself, "That Vulcan would have us killed for his precious science!" after Spock instructs the security detail to capture the Horta.
This episode was originally scheduled to be filmed before Star Trek: This Side of Paradise (1967), with Ralph Senensky assigned to direct it, but during pre-production the two episodes and the directors were switched because Gene L. Coon thought "Devil" would be a tough assignment to first-time Trek director Senensky.
The only interior portion of the Enterprise seen in this episode is the bridge, and that in only two scenes - when Scotty speaks with Kirk about replacing the circulating pump, and at the end, after the landing party has returned to the ship.
The clubs used by some of the Janus VI colonists during their hunt for the Horta appear to be of the same design used by Kirk during his fight with Spock in the transporter room in Star Trek: This Side of Paradise (1967).
Gene L. Coon's original script called the planet Thetis Six. Researcher Kellam de Forest pointed out that "The name 'Thetis' has already been assigned by astronomers to the 17th largest asteroid discovered in 1852 in the asteroid belt between Mars and Jupiter," and suggested the name Janus VI instead.
McCoy specifies that he had the Enterprise beam down about a hundred pounds of thermal concrete. In James Blish's novelization of the episode in Star Trek 4, which was generally based more so on early draft scripts than final drafts, states that McCoy only used ten pounds of concrete.
The scene of Kirk, Spock, and McCoy discussing the silicon spheres and the murders was originally set in the Enterprise briefing room, but when the death of William Shatner's father halted production, it was re-written to be set in Vanderberg's office instead, so moving to a new set won't slow the filming even more.
Although the uniforms come in various colours, Vanderberg is the only miner who wears a yellow specimen, and Ed Appel uniquely wears a purple one, which apparently was later worn by the dead Tellarite on Memory Alpha in Star Trek: The Lights of Zetar (1969), as well as on Ozaba in Star Trek: The Empath (1968).
In Star Trek VI: The Undiscovered Country (1991), the Klingon moon Praxis explodes because of two much mining. Between the Human mining and thousands of Horta leaving tunnels tunnels wherever they go, it seems likely that Janus 6 will eventually explode.
Shortly before shooting the scene in which the Enterprise security guard is killed by the Horta, William Shatner was informed that his father had died. His mournful demeanor as he knelt down by the remains of the guard was genuine.