In Justman and Solow's book "Inside Star Trek - The Real Story", the goes that Fred Frieberger had Shatner, Nimoy and Roddenberry in his office to decide who is ultimately the star. Both Shatner and Nimoy claimed the honor. Gene, who desperately wanted to avoid the confrontation, stalled until, pressed for an answer, he said angrily, "It's Bill. Bill is the star of the series," and bolted from the room.
Years later, Shatner would repeatedly defend Frieberger's rep, but Nimoy was always down on him.