There were really three groups of actors on the original Star Trek series:
The leading actors: William Shatner, Leonard Nimoy, and Deforrest Kelley.
There were the weekly and occasionally recurring guest stars. These were people like Richard Kiel (Jaws in the Bond films), Joan Collins, Roger C. Carmel (Harry Mudd), etc.
But the strength of the show is often measured in the strength of its supporting regular cast. Scotty (James Doohan), Uhura (Nichelle Nichols), Sulu (George Takei) and Checkov (Walter Koenig) were often used as set dressing and had little to do. But when they had something to do, they did it well and memorably.
Here are some facts about the actors that gave space for Shatner’s overacting: