The episode was being rewritten as it was being shot. Cast members recalled Gene Roddenberry sitting under a tree, frantically reworking the script to keep it both under budget and within the realms of believability. As a result the filming went over schedule and took seven days instead of the usual six.
William Blackburn (a professional ice skater in real life), who played the White Rabbit, got the costume from Ice Capades for free. The claustrophobic Blackburn had a really painful time wearing it, especially as costume designer William Ware Theiss had originally sewn the Rabbit head to the suit. After nearly suffocating, Blackburn tore off the head, for which Theiss became very mad at him. Finally, they negotiated and Theiss put the head back with Velcro. Afterwards, Blackburn had no problem with the costume. He also commented that wearing the Gorn head in Arena (1967) was "even worse."
A chained tiger was brought in to appear in the episode, but never directly interacts with any of the performers. William Shatner had originally hoped to wrestle it, but was persuaded that it would not be a wise decision.
Editor Fabien D. Tordjmann came up with the idea of having Finnegan continue to pop out of nowhere as Kirk pursues him. This gave the impression that there was more than one Finnegan, or at least that he was not quite human.
This is the only episode in which the U.S.S. Enterprise is seen orbiting a planet from right to left. The shot was deliberately reversed in post-production because the shape of the Eastern United States and the Caribbean sea could clearly be seen on the globe used as a model for the planet.