The title is taken from Adonais: An Elegy on the Death of John Keats by Percy Bysshe Shelley. Line 415 reads "Who mourns for Adonais?" Shelley's Adonais is derived from Adonis, a male figure of Greek mythology associated with fertility. Also, "Adonais" would be the English plural of the Hebrew Spoken Name of God (the Hebrew word 'adonai' simply means 'lord'), so it would mean "Who Mourns for the Gods?"
The producers were looking for someone with an English dialect and Shakespearean theatrics to pull off the Apollo role. First, they wanted to find someone in England, but rather decided to look for an actor at the San Diego Shakespeare festival. The head of the theatre recommended Michael Forest, who was already in Hollywood making films at the time. Forest was called in for an audition, where he first had to take off his shirt to let them see if he had the muscles needed for the part. Next, they asked him to read some lines in a British accent. Forest refused, claiming he couldn't do it, but is able to speak in a Mid-Atlantic accent, probably more suitable for the character. He did it, and they gave him the role.
In the original script, the gods and other mythological figures were mentioned in their Latin names, but, in the revised final draft (and the finished episode), they are called by their Greek equivalents (possibly at the suggestion of series researcher Kellam de Forest).
Michael Forest recalled working with his co-stars, "Leslie Parrish was a delightful person to work with; no problems; never any difficulties; we would just discuss what we were going to do and we would do it. She was excellent and very personable. William Shatner was a bit of a problem, however. You never saw me standing with him; we were always in different shots. We would be talking to one another, but we wouldn't be on camera at the same time. I'm sure that's what he stipulated - because I was so much taller."
William Shatner was so concerned with the height disparity that he disallowed any shots which would show him and the much taller Michael Forest side-by-side in the same frame. According to Forest, whenever Shatner would speak to him, he'd notice him inadvertently standing on his tip toes.