THE ENEMY WITHIN was the fifth episode of the original STAR TREK television series to be broadcast. The date was October 6th, 1966. It's the one where, thanks to a transporter malfunction, Captain Kirk is split into two separate beings, a good Kirk and a bad Kirk. While two Kirks on the Enterprise are trouble enough, there's a landing party on the planet surface who are about to freeze to death thanks to said transporter malfunction. What, Starfleet didn't have shuttle craft at this point in time? Kirk and Spock forgot about them?
At the pre-production meeting, the cast and production crew were sitting around a table at the Desilu/Paramount Studios offices for a read through of the Richard Matheson script.
What a producer said:
"We've got a tight budget on this week's episode. No guest stars and we'll use the existing planet set. Bill, you get to do double duty. You're playing good Kirk, bad Kirk. We'll need you to really give it some depth when you're playing the bad Kirk. Oh, and by the way, when you're bad Kirk you're going to be wearing eyeliner, just to give you a more sinister look."
What Shatner heard :
"Turn it up to eleven."