Leonard Nimoy’s situation as Spock was structurally unusual. He had survived the transition from the first to the second pilot; he was the only principal cast member who had. The character he was playing was, in its second-pilot version and in the early episodes of the series, still being defined — Spock’s specific emotional register, the precise nature of his Vulcan heritage and its behavioral implications, the relationship between his logic and his suppressed human half were all worked out during production rather than being specified in advance.
Nimoy has described the development of Spock as a collaborative process — his own performance choices, Roddenberry’s writing, and the feedback from early episodes all contributing to the character’s crystallization. The hands-clasped-behind-back posture, the slight forward tilt of the head when processing information, the specific flatness of vocal delivery for declarative statements — these were Nimoy’s choices, made within the character framework Roddenberry had established.
DeForest Kelley as Dr. Leonard “Bones” McCoy was not part of either pilot. He was cast for the series specifically, joining after both pilots had established the basic structure. Kelley was known primarily as a villain actor in Westerns — he had played numerous heavies over the previous decade — and his casting as the warmly humanistic ship’s doctor represented a significant departure from his established type. The character he played became the third point of the central triangle: where Kirk balanced action and thought, and Spock represented pure logic, McCoy represented emotion and intuition, the voice that insisted on the human cost of decisions made in the abstract.