Gene Roddenberry made it clear to William Shatner and Leonard Nimoy who was the lead of Star Trek

Star Trek: The Original Series was always meant to be an ensemble show with William Shatner, Leonard Nimoy, and DeForest Kelley as the triumvirate. Though Shatner was the star of the show, that didn't mean he would get all of the big scenes or every episode would be solely about him. As Spockmania grew, more fans were tuning in for Leonard Nimoy's depiction of Spock, which caused some issues among the actors. By August 1967, tensions had grown between the stars of The Original Series to the point where Gene Roddenberry felt he had to step in.

In a scathing letter, he made it quite clear that Shatner was the lead of the series, but he also made it abundantly clear that he would be Roddenberry's version of the lead and not Shatner's. Roddenberry clarified Nimoy as "a strong, effective, and integral second lead." So, in sounds like there really were two leads to the series, and they functioned as a unit.

Over the years many actors have had more than a few things to say about the tensions on set as well as Shatner himself, but in Roddenberry's letter, dated August 17, 1967 [via The Fifty Year Mission The First Twenty-Five Years by Edward Gross and Mark A. Altman], the creator of the series left no doubt as to who was in charge and how things would be run.

Whether nor not Roddenberry's intervention made a difference on the set is not really clear, especially since the opinions of Shatner didn't seem to change over the years. But there was one thing the letter made sure to point out and that was Roddenberry was sick of what was happening on the set, and he wasn't going to take it anymore. Writer David Gerrold, in The World of Star Trek, did indicate that, after the letter was sent out, things calmed down on the set, even if only slightly. And Shatner and Nimoy did have a long-running friendship so what happened on the set seemed to stay on the set.