“The approach we were told to take is that Kirk really had to be the one to lead everyone. […] Not necessarily that he had to actually have the idea to do something, but it had to appear as if he has the idea.”

Krikes recalled how illogical the mandate was, and how, when you watch the film, you can see how Kirk has been arbitrarily inserted into various scenes where he doesn’t belong. For example, there’s a scene near the end of the movie in which Spock (Nimoy) has a conversation with his father Sarek (Mark Lenard). Kirk is present in the background, watching the conversation. There is no reason for him to be there. But, golly, that’s what Paramount wanted.

…

“I think the perfect example in the movie is when Spock goes into the belly of the Bird of Prey to use the computers and learns that the sound is whale songs. It’s Kirk who has the idea to go back through time, although Spock is the one who plants the suggestion in Kirk’s mind. Kirk verbalizes it, and that’s the way it had to be played. We were told Bill had to be the leader at all times. In that scene, if you’re reading it, you say, ‘It’s Spock’s idea,’ but on film, Spock’s discovery that it’s humpback whales is not as important as Kirk’s idea of going to get them.”