Because at the time, Shatner was a damn fine SERIOUS actor. The next time its on TCM, watch "The Brothers Karamazov". In it, he more than holds his own against serious actors like Lee J Cobb and Yul Brynner. He is absolutely brilliant in 90% of the first season, and in a good deal of the second. In the latter part of the second, and all through the third, he starts having to deal with not only lesser scripts, but also shorter production times, lesser directors (no Pevney and Daniels), and increasing fatigue.
Enemy Within? It's one of his best! The negative Kirk is SUPPOSED to be totally out of control, almost animalistic. Shatner absolutely has a talent for that sort of thing, and exploits it for its worth. While at the same time, as the so-called 'positive' Kirk, you can see him all through the show, with his words and actions, a man searching within himself for something thats not quite there, all while trying to deal with a crisis. He also manages to have subtle moments as Kirk(-) - the scene in sickbay when Kirk (+) takes his hand and manages to strengthen both of them. Kirk(-) gives a grateful smile, but you can also see the scheming thoughts he's having. The best one is the one near the end where, after knocking out Kirk(+), and going to the bridge, he encounters Rand, apologizes to her, telling (real) story, then making a dinner date with her. He goes to the effect of 'meet you at 8' enters the lift, and just as the doors close, makes a SUBTLE, evil leer. Chilling! (The original script, btw, had him pounding the lift walls in triumph - they wisely cut that out.)
out of here!"
