In "Albatross", Spock breaks McCoy out of prison to find a cure for a disease which is killing the Enterprise crew. The Vulcan reminds him that if he goes and fails, he will also die. Bones' response? "I'm a doctor, Spock. A doctor. Get us beamed aboard!" Not since "The Empath" has one of McCoy's reminders about his profession been so heart-meltingly awesome.
As is his joy and relief when he finally gets proof that he didn't inadvertantly cause the previous plague.
In "The Pirates of Orion", McCoy's attitude around the terminally ill Spock.
Thelin: True. A warrior race has few sympathies, but one we do possess is for family. In your time plane, you will live and so will your mother. That is valuable. [gives Spock the Vulcan salute] Live long and prosper in your world, Commander Spock.
Spock:[returning the gesture] And you in yours, Commander Thelin.
Behind the scenes, Leonard Nimoy drug his heels on returning to the role of Mr. Spock. Not because of any antipathy to the character, but because Filmation (a notoriously budget-conscious company) didn't want to hire on George Takei, Nichelle Nichols or Walter Koenig note The plan, as far as can be determined, was: hire the three main stars (Shatner, Kelley and Nimoy), and then hire on Majel Barrett and James Doohan to essentially play everyone else save for a very occasional guest star. Nimoy essentially told Filmation "If you don't get Sulu, Uhura and Chekov, you don't get Spock." In the end, Filmation and Nimoy came to a compromise. Takei and Nichols were hired, and Koenig was guaranteed an option on a script for the series, eventually turning in the episode "The Infinite Vulcan".