The plot point about Kukulkan actually being an alien life form was inspired by the Erich von Däniken book Chariots of the Gods, especially claims like Indian sites in Peru having actually been created by aliens as methods of communication.
In the portion of the episode when Uhura expresses concern about the condition of "the captain and the others," Spock replies curtly, "Return to your duties," writers David Wise and Russell Bates worked for almost two hours trying to write a tart retort for her to one-up Spock but couldn't. So Wise wrote a place holding "slug line" of "Pointy-eared fool!" with the plan to fix it later. They forgot to fix it and turned in the script and it made it all the way to Nichelle Nichols' recording session. When she read the line silently she looked up and asked, "Am I really supposed to say that?!" Wise and Bates told her "No!" So the line was not recorded nor any replacement line.
Authors Russell Bates and David Wise deliberately modeled their plot on Who Mourns for Adonais? (1967), written by Bates' friend Gene L. Coon.
The title is based upon a quote from William Shakespeare's play "King Lear" (Act I, Scene IV): "How sharper than a serpent's tooth it is to have a thankless child!"
The character of Dawson Walking Bear was "borrowed" for this episode. He originally appeared in "The Patient Parasites," a script that was never produced. This episode features him more prominently than that script did and also gives him a unique personality. His first name, "Dawson," is not used in this episode. The presence of Ensign Dawson Walking Bear in this installment means it was the first episode to feature the appearance of a Native American starship crew member on Star Trek.