- Creator Backlash: Harlan Ellison was so annoyed by the changes to his script that he tried to take his name off it.
- Creator's Favorite Episode: Gene Roddenberry named this as one of his ten favourite episodes, William Shatner named it as his second favouritenote and Leonard Nimoy named it as one his top five. Yeah, this episode's pretty damn good.
- Edited for Syndication: Rodent's death is deleted in some rebroadcasts, but is intact in home video editions. When McCoy meets Rodent holding the milk bottle, the scene ends with McCoy collapsing, then cuts to McCoy meeting Keeler in the Mission. In the complete scene, after McCoy collapses, Rodent picks McCoy's pocket and takes his hand phaser (which he took from the transporter chief) and accidentally sets it on overload, killing himself.
- Inspiration for the Work: Harlan Ellison was inspired by reading a biography of evangelist Aimee Semple McPherson and thought that it would be an interesting idea to have Kirk travel back in time and fall in love with a similar woman of good intent, but someone who must die in order to preserve the future. Ellison considered that it would have a heartrending effect on Kirk.
- Playing Against Type: Joan Collins, both at the time (she made her name playing ingenue bombshells in The '50s) and in retrospect (her most famous role is as the amoral, conniving diva Alexis from Dynasty (1981), just about the polar opposite of Edith Keeler in every way). Collins herself likes to comment on her Typecasting as a villainess by reminding everyone that she played Edith, who to be fair remains one of her most famous roles. Ironically, in a 1983 interview in Playboy, Collins had difficulty remembering the details of her role, even when the interviewer reminded her of her character's romance with Kirk.
- Recycled Set: The New York streets that Kirk and Edith walk down are the Mayberry set from The Andy Griffith Show.
- So My Kids Can Watch: Joan Collins' story at the Star Trek 30th Anniversary of how she got the part of Edith Keeler:In 1967, when my two children finally entered school, I decided I wanted to go back to acting, and soon afterwards, my agent Tom Corman called to say I'd been offered a great part in a Star Trek episode.
"Star what?" I said?
"It's a huge new cult show," said Tom. "Obviously you haven't been reading the trades."
"No, I've been too busy reading Mother and Child Care by Doctor Spock."
"Forget Doctor Spock," he said. "Start thinking Mister Spock."
"Ah, yes!" I said. "Mister Spock, the one the ears? The children love that show!"
"Right, then you'd better do it. You'll probably be queen of the universe, possess intergalactic powers, wear tight, revealing costumes. Trust me, I'm your agent."
A week later I was cast as Edith Keeler, a saintly Earthling, who works as a social worker in a 1930s mission for down-and-out bums in New York's Bowery. Thanks, Tom.
- Troubled Production: The episode may be a contender for the high point of the entire Star Trek franchise, but it had a troubled time getting there:
- Harlan Ellison's first draft was agreed by just about everyone to be a masterpiece in its own right, but didn't really feel like a Star Trek episode, with Gene Roddenberry's chief complaint being the inclusion of a drug-dealing character who helps get the plot of the episode underway, along with Kirk having him executed via firing squad in the episode's climax. Ellison figured that even in the future a military outfit like Starfleet would have some unsavory characters. On top of that, Ellison added in an element of barely-suppressed racial undertension between Kirk and Spock, even though Kirk making such remarks had served as an instant Out-of-Character Alert in "What Are Little Girls Made Of?" earlier in that season. It should be noted that in 1966 it is highly unlikely that NBC's censors would have allowed the drug-dealing aspect to have been televised, anyway, even though it was emphatically Drugs Are Bad.
- The job of rewriting Ellison's script was given to script editor Steven W. Carabatsos, only for the resulting script to turn out so awful that it nearly resulted in Ellison quitting the project in fury, and played a major part in Carabatsos being let go from the series and replaced by D.C. Fontana shortly afterwards. Ellison then went back and did another rewrite himself, with input from producer Gene L. Coon, and despite deleting the racial elements and changing the drug dealer's demise to a Karmic Death inflicted by the Guardian of Forever, Roddenberry still wasn't happy with it.
- At this point, Fontana herself took a shot at the script, essentially starting over using Ellison's basic story outline as a start point. This time, everyone involved agreed that Fontana had absolutely nailed the story, with only a few small rewrites subsequently being done by Roddenberry and Coon, mostly to account for actor, set and prop availability.
- Filming was comparatively more smooth, though the demands of the shoot meant they needed an extra two days to film everything. Production designer Matt Jefferies was also laid low with a flu virus in pre-production, resulting in his supervisor, Rolland M. Brooks having to design the Guardian of Forever.
- Before the episode was broadcast, Roddenberry publicly badmouthed Ellison's work on the episode, which infuriated the writer and caused him to demand that he be credited under his pseudonym, "Cordwainer Bird". Since it was already widely known even in 1967 that he used this to flag works which had been wrecked by Executive Meddling, and that this would cause viewers to expect the episode to suck even before watching it, Roddenberry used every means he could to drag out the Writer's Guild arbitration process until the episode was ready to air, and it was too late to do anything more about it. (Rodenberry also wanted to avoid the "Bird" credit because he feared it would discourage good sci-fi writers from contributing to the show.) However, this also meant that Fontana, who should have been credited as co-writer, ended up having to go without credit. In fact, Fontana's identity as the person who wrote the final teleplay wasn't revealed to Ellison (and, by extension, the general public) until many years later, when he wrote his tell-all book about his experiences writing the episode. (Until then, the assumption had been that Roddenberry, Coon, or all the staff writers by committee had rewritten it.)
- Wag the Director: According to Harlan Ellison, William Shatner counted how many lines he had in the script and demanded a change when Leonard Nimoy had more lines than him.
- What Could Have Been:
Spock: You could not stop Beckwith. I understand that. But Beckwith Amoral, evil, a killer, selfish and capable of anything. Why
- Harlan Ellison's script was heavily altered to fit the tone of the series (as well as to trim a large cast and settings that were far beyond the show's budget). Rather than McCoy, the past was changed by an evil drug-dealing crewman who ends up in a particularly hellish And I Must Scream situation trapped inside a newborn star. Also, Kirk would have been unable to let Edith die, and Spock would have had to step in to prevent an irredeemable original character from saving her. Once they returned to the Enterprise, this remarkable conversation would have ensued:
Kirk: Why did he try to save her, at risk of his own life?
Kirk: We look at our race, this parade of men and women, and the unbelievable harm and cruelty they do. And we sigh, and we say, Perhaps our time is past, let the sharks or the cockroaches take over. And then, without knowing why, without even thinking of it, the worst among us does the great thing, the noble deed, that spark of impossible human godliness. And we say, Perhaps the human race is entitled to a little more sufferance. Let them keep trying to reach the dream.
Spock: Evil can come from good, and good from evil. But the little man Trooper
Kirk: He was negligible. He fought at Verdun, and he was negligible. And she...
Spock: No, she was not negligible.
Kirk: But... I loved her...
Spock: No woman was ever loved as much, Jim. Because no woman was ever offered the universe for love.
- In the original story, Kirk and Spock are aided in the 1930s by a legless vagrant called Trooper who reveals himself to be a veteran of the Battle of Verdun. Trooper is killed by Beckwith (the drug-dealing crewman) during the episode, but unlike with Keeler, the Guardian indicates that Trooper's fate has "negligible" impact on the timeline, to Kirk's distress.
- The original finale also included a You Called Me "X"; It Must Be Serious moment when Spock calls his captain "Jim" for the first time, then gently invites him to come to Vulcan to rest, saying it is peaceful there, "the nights are very long. In the morning, there is the sound of silver birds against the sky." Most fans encountered this poetic line via James Blish's adaption. Fan Fic about Vulcan often includes those silver birds; there have even been songs written about them. The epic Kraith series by Jacqueline Lichtenberg has Kirk at last accepting that invitation, in "Spock's Mission."
Trivia / Star Trek S1 E28 "The City on the Edge of Forever"