HERB: In August, before the series debuted, a screening for NBC was held at Desilu, at which time the premiere episode was to be selected from the small group of episodes that could be ready in time for the first airdate, Thursday night, September 8, 1966, at 8:30 p.m. EST. (Are you still with us, trivia fans?)
NBC’s and Desilu’s desire was to deliver what the opening main title promised: “strange new worlds.” There were really only two serious candidates: “Man Trap” and “The Naked Time.” The second pilot, “Where No Man Has Gone Before,” was held back because it was too expository in terms of the series concept and characters, a problem with most pilots telecast as series episodes. “Where No Man” was necessary for selling, not necessarily for televising. “Mudd’s Women” was out of the running because the opening-night critics would have had a field day with the story of “space hookers in the galaxy.” “Charlie X” was too gentle a tale, dealing with the problems of a teenager. “The Enemy Within” was another shipboard show that lacked the scope of Star Trek's premise. Despite a fine performance by William Shatner, it was held back for later telecast.
BOB: I felt that “The Naked Time” made it easy for viewers to understand the main characters of our show and their relationships to each other. But the story took place mainly aboard ship. I suspected the NBC people wanted “Man Trap,” because it was scarier and more exploitable than the others. I made a speech in the projection room pushing for “The Naked Time,” but Gene was strangely silent and so was Herb. I suspected Herb had decided to give the network what it wanted and had cautioned Gene about making waves— but he hadn’t cautioned me. I should have saved my breath. I had forgotten the “golden rule”: “Those who have the gold, rule.”
My recommendation was ignored, and NBC made the decision. Roddenberry and Solow agreed with them. The first show broadcast would be “Man Trap,” the fourth episode filmed. It not only took place on a distant planet, but also featured a character, Nancy Crater, who had been replaced by a loathsome salt-sucking creature. I later realized NBC was right. With “Where No Man Has Gone Before” out of contention, “Man Trap” was the only viable “new worlds” premiere episode available. Later, prior to the premiere broadcast, NBC research-tested “Man Trap” at Preview House. The results confirmed NBC’s and Desilu’s decision to lead off with the loathsome salt-sucker.
Inside Star Trek, p.162–163