Crewman DePaul is played by Sean Kenney, who portrayed the injured Captain Pike in The Menagerie: Part I (1966) and The Menagerie: Part II (1966).
According to David Gerrold, the computer tallies of war dead in this episode was a statement about Vietnam War deaths that began to be registered on nightly newscasts in 1967.
Scotty's refusal to lower the shields against orders is based on an actual story from James Doohan's military service. As a lieutenant in the Royal Canadian Artillery, he was threatened with court martial for real for saying "No sir, I will not," to a visiting colonel when he realized a training exercise order would entail blowing the heads off some of his own men. Fortunately, his immediate superiors backed him up and, like his fictional character, he was eventually promoted to captain.
In his memoir, "Beam Me Up, Scotty," James Doohan described Gene Lyons (Ambassador Robert Fox) as being "out of his element" and "completely discombobulated" during filming. He added that it took Lyons many takes to get his lines right and that they finally "went to having him speak off-screen." Doohan speculates that Lyons, who was an experienced actor, may have been thrown off by the science fiction element as such shows were relatively rare at the time.
Another beautiful matte painting was created for this episode by Albert Whitlock. Unfortunately, it is the last painting in the series into which live actors were inserted. This matte of the Eminiar city was re-used as the backdrop of Scalos in the third season episode Wink of an Eye (1968).