I liked John Hoyt as the doctor, though if he'd been kept on, we wouldn't have gotten DeForest Kelley as McCoy. Hoyt was a good choice I thought, as he had sci-fi experience (the wheelchaired bad guy in When Worlds Collide
, and had appeared as an alien in The Outer Limits: The Bellero Shield, with Sally Kellerman about a year before "The Cage"). I watched "The Menagerie" recently, and most of the nameless supporting characters in the "Cage" footage were memorably unmemorable.
The only thing I found notable about Peter Duryea was his voice sounded a bit like his dad's (Dan Duryea).