DeForest Kelley refused a line of dialogue
Star Trek II: Wrath of Khan had one of the most, if not the most, emotional scene ever seen in a Star Trek movie—the death of Spock (Leonard Nimoy). Those powerful, gut-wrenching moments include some of the best acting by Nimoy and William Shatner (Admiral Kirk) as well as James Doohan (Scotty) and DeForest Kelley (Dr. Leonard McCoy). McCoy is powerless to help the green-blooded Vulcan the one time he wanted to help him the most. There was nothing anyone could do but watch as Spock passed away. And the script originally called for Kelley to make the declaration he’d made several times on Star Trek: The Original Series.
"“He’s dead, Jim.”"
Kelley didn’t want to say the line of dialogue because he thought the audience would laugh at the catchphrase, which would lessen the impact of Spock’s death. His protest didn’t fall on deaf ears, and the line was transferred to James Doohan who put his own spin on it when he said, “Sir! He’s dead already.”
DeForest Kelley was right
Spock’s death scene was incredibly powerful and painful, and some might disagree with Kelley’s assessment that the audience would have laughed. But it was so much better not to take that chance than to risk ruining the scene with a chuckle or two.
There’s a time for a comedic line, and Bones’ catchphrase has become enshrouded in comedy. There is no avoiding that people laugh at it now. So what the scriptwriter was thinking by including the line in Wrath of Khan is beyond me. I’m just glad Kelley was wise enough to know what could potentially happen if he uttered it.