What he really should have said is: "Prepare for every other episode of television you will ever watch to seem like it's just weakly trying to measure up to the awesomeness of 'The Enemy Within.'"
Starring William Shatner and William Shatner as James T. Kirk: Shatner seriously out-Shatners himself in every single moment of this episode, as Good Kirk agonizes over existential angst with long, brooding Captain's Logs, and Evil Kirk flings himself around the ship swilling brandy, beating the crap out of people, using a tub of makeup concealer that Kirk apparently keeps on his vanity, and yelling such gems as "I'M CAPTAIN KIRKKKKK!" and "IT'S MY SHIP!" Did they even have Emmys back then? If not, someone needs to go back in time, invent the Emmys earlier, and then GIVE ONE TO WILLIAM SHATNER.
Spock: VULCAN NERVE PINCH! YAY! Spock spends the non-nerve-pinching remainder of the episode deeply concerned with mutiny. At first, he tells Kirk that he can't let the crew see Kirk as imperfect or he will lose command (what are we, operating on Klingon rules now?). Then, he tries to take over command himself a few times, because he's concerned about Good Kirk's inherent weenieness. He also awesomely says: "If I seem insensitive to what you're going through... it's the way I am." HA.
McCoy: "He's dead, Jim." First time, and he's talking about the crazy dog-with-horns thing that is in this episode for reasons that are NEVER EXPLAINED. Where did this animal come from? Why were they beaming it up to the ship in the first place? Why does everyone persist in carrying it around for the entire episode like Paris Hilton?
Scotty: We've got an engineering set! Scotty spends almost no time there in this episode, though. Actually, no one seems to spend any time there on this ship, because it's completely unmanned. This, to me, is a bad idea.
Sulu: If this episode is pretty much 95% Shatner being AMAZING, the other 5% is "Sulu having the weirdest dialogue of all time." Every single thing he says is some sort of quip, though my favorite one is definitely: "I think we'd better call room service again. That coffee's taking too long." Thank you, script writer Richard Matheson...?
I Dream of Janice Rand: Continuing the trend of every Janice Rand scene being incredibly awkward in some way, she is first molested by Evil Kirk (Evil Kirk kisses with tongue, by the way), then swears to Kirk and Spock that she wouldn't have told anyone or gotten the captain in trouble for molesting her, then gets totally busted by Spock at the end for being a little bit turned on by the whole thing. Well, okay then!
The Burning Question: "Why THE HELL doesn't somebody just SEND A FREAKING SHUTTLECRAFT to pick up THE LANDING PARTY?" That drove me NUTS every other time I saw this episode, because I didn't know that it was so early in the series and we'd never even seen a shuttlecraft before. Now I'm cool with it, because I'm assuming that all of the nonexistent engineers from engineering are busy building the shuttlebay.
Ship > Everything: This episode is all about good vs. evil in the nature of man. Good represents compassion, intelligence, and love. Evil represents sex, violence, and Kirk's overpowering need to go completely apeshit on anyone who tries to take the Enterprise from him.
WTF Prop of the Episode: I'd really love to say "THE DOG," but I'm going to have to go with the spray bottles and dustbuster that McCoy uses to treat the geologist's hand wound in sickbay. WHAT!? I mean, there's old country medicine, and then there's SPRAY BOTTLES AND A DUSTBUSTER. This leads me to the other burning question of the episode: Why is the ship's liquor cabinet in Sickbay, anyway?
You would rather spend an evening with:
Sulu, for the random one-liners