If any character should have been cast differently, though, it should have been Freeman - Paul Baxley was Shatner's stand-in, and it shows in his flat performance and line delivery.
Stunt double actually, not stand-in. Eddie Paskey was Shatner's stand-in. Baxley was probably cast because of the upcoming fight scene.
I think Baxley stands out in the episode because he's behaving exactly like a real person, which is to say flat and uninteresting, while the industry-standard acting style of the period was to put on more of a deliberate personality. Professional actors in 1967 played it a little bigger, while Baxley might have fit right in doing an episode of re-imagined BATTLESTAR GALACTICA circa 2005, with it's understated naturalism.
I'm phrasing it badly, but I think I'm onto something.