Yeah, you can see that in the early episodes, how each one was written around a central guest star. In the pilot it was Landau, of course. "Memory" was written around Albert Paulsen's character. "Operation: Rogosh" centered on Fritz Weaver as the titular villain. "Old Man Out" showcased Mary Ann Mobley (to my great enjoyment). But once you get to things like "Wheels" and "A Spool There Was," they're getting written around Rollin again.
Star Trek kind of went the same way. Roddenberry overtly said he planned to emulate Wagon Train's anthology-like focus on guest stars of the week, and you can see that with things like "Where No Man..." focusing on Mitchell and Dehner, "The Corbomite Maneuver" centering on Dave Bailey's arc, "Mudd's Women" centering on Harry and Eve, and "Charlie X" being self-explanatory. But then Spock became the breakout character and the stories started to emphasize him more, with Roddenberry and Shatner insisting on keeping Kirk equally central, and McCoy becoming the third member of the triumvirate by virtue of his relationships to the other two. So it became more focused on the leads than on the guest stars as originally planned.
Coincidental that it happened with Landau and Nimoy simultaneously, given that Nimoy replaced Landau on M:I, and given the longstanding but now-debunked rumors that Landau was offered the role of Spock before Nimoy. (I think it was more that he was on a list of alternate candidates, but Nimoy was always the first choice.)