In several interviews, Morgan Woodward noted that his work on the episode greatly affected him on both a personal and professional level. Woodward felt the part of Van Gelder was perhaps the most physically and emotionally exhausting role he played. He also stated his experience in playing the part resulted in his being in a largely anti-social state of mind for a few weeks following. However, Woodward, who would later play Captain Tracey in The Omega Glory (1968), credits his work on Star Trek in helping him to finally break away from his being typecast in Western roles.
The part of Helen Noel was originally written for Janice Rand. However, the producers wanted to avoid showing Kirk becoming involved with her, and Grace Lee Whitney was already on the verge of leaving the show due to personal problems on the set. Whitney had been sexually assaulted by one of the show's executives at a wrap party. She believes she was pushed out for speaking up in protest of this. In any event, from a dramatic point of view, it made more sense for a trained psychotherapist, rather than a yeoman, to accompany Kirk to the Tantalus rehabilitation colony.
James Doohan and George Takei do not appear in this episode. Scotty appeared in the original script, operating the transporter in the first scene, when Van Gelder is beamed aboard. His appearance was nixed by Robert H. Justman, who saw this as a way of saving costs by eliminating Doohan, who would have been paid $890 for the episode ($8,300 in 2023), and replacing him with a random performer (Anthony Larry Paul, playing Lieutenant Berkeley), hired for a much lower salary.
A shipping label produced for Star Trek: Deep Space Nine (1993) shows that a (now elderly) Dr. Van Gelder is still in charge of the Tantalus Penal Colony in the 2370s.
To be fair, he is more likely to be the son or even grandson of the original Dr. Van Gelder given he would have to be at least 153 years old by the time of DS9.
Lethe (Susanne Wasson) was named after the river in Hades in Greek myth. Those condemned souls who drank from it forgot their past lives. Explaining her blank, vegetative stare from neutralizer "treatment."