The voice of Sargon was provided by James Doohan (Scotty).
John T. Dugan wrote the story under the pseudonym of "John Kingsbridge," with Roddenberry doing the teleplay.
Footage of a test android was filmed but not used. A clip of it appears in the end credits of the episode "The Immunity Syndrome."
Spock: Someone, or something, is attempting to attract our attention.
Kirk: Someone, or something, has succeeded!
Sulu: Planet dead ahead, Captain. Becoming visual.
Spock: Class M planet, Captain.
Kirk: Close to Earth conditions.
Spock: With two very important exceptions: it is much older than Earth, and about a half million years ago its atmosphere was totally ripped away by some sort of cataclysm. The planet has evidently been dead since then. Sensors detect no life of any kind.
Sargon: ALL YOUR QUESTIONS WILL BE ANSWERED IN TIME, CAPTAIN KIRK...
Kirk: The planet is dead... there's no possibility of life there, as we understand life...
Sargon: AND I AM AS DEAD AS MY PLANET. DOES THAT FRIGHTEN YOU, JAMES KIRK? FOR IF IT DOES... IF YOU LET WHAT IS LEFT OF ME PERISH... THEN ALL OF YOU, MY CHILDREN... ALL OF MANKIND... MUST PERISH, TOO.
Sargon: (Sargon has placed his mind within Jim Kirk's body.) Lungs filling with air again... to see again... heart pumping, arteries surging with blood again... after a half a million years... to be again!
Sargon: The records of our travels were lost in the cataclysm which we loosened upon ourselves.
Kirk: A war?
Sargon: A struggle for such goals, and the unleashing of such power, that you could not comprehend...
Kirk: Then perhaps your intelligence wasn't so great, Sargon. We faced a similar crisis in our early nuclear age. We... found the wisdom not to destroy ourselves.
Sargon: (patronizingly) And we survived our primitive nuclear era, my son. But there comes to all races an ultimate crisis... which you have yet to face.
Kirk: I don't understand...
Sargon: One day our minds became so powerful, we dared think of ourselves as gods!
Kirk: They used to say, if man could fly, he'd have wings. But he did fly; he discovered he had to. Do you wish that the first Apollo mission hadn't reached the moon, or that we hadn't gone on to Mars and then to the nearest star? That's like saying you wish that you still operated with scalpels and sewed your patients up with catgut, like your great, great, great-grandfather used to do... Dr. McCoy is right in pointing out the enormous danger potential in any contact with life and intelligence as fantastically advanced as this. But I must point out that the possibilities--the potential for knowledge and advancement is equally great. Risk--risk is our business.
Thalassa/Mulhall: I require only your silence. Only you and I will know that Dr. Mulhall has not returned to her body. Isn't that wroth your captain's life?
McCoy: I will not peddle flesh! I'm a physician!
Thalassa/Mulhall: A physician? In contrast to what we are, you are a prancing, savage medicine man. You dare defy one you should be on your knees worshiping?