In William Shatner’s Star Trek Movie Memories, the director of Star Trek V outlined his original plot for the film. This more serious take featured Spock and McCoy following Sybok, leaving Kirk alone to confront “God” and the Vulcan.
Nimoy outright refused to appear in the proposed movie, saying that there was no chance at all that Spock would turn against the man who had sacrificed so much for him in Star Trek III. Shatner tried his best to argue with his co-star and friend, but Nimoy was steadfast: “pain or no pain, brother or no brother, Spock would not betray Captain Kirk.”
The same day, DeForest Kelley came to Shatner and stated that had the same problems with the script: under no circumstances would McCoy turn his back on Kirk.
Citing both actor’s knowledge of their own characters, Shatner began to make changes to the plot outline. While he writes that he doesn’t know whether or not those changes were the best thing for the final product, Shatner admits that if someone else had written a script where McCoy and Spock turn against Kirk, he would have raised the roof over it himself.
This is the very definition of loyalty.