The lore of Star Trek is impressive and expansive, but the television and films tell an amazing story. Sometimes it's the stuff behind the scenes that commands the most attention. We hear all the time about how certain figures in Star Trek wanted something one way and spoke up and got it, and those figures are usually the same ones. The Patrick Stewarts and William Shatners.
Yet, sometimes those you don't think about much as having changed a core concept of a film or show can pop up, taking us all by surprise. One of the big names who apparently had a major hand in how his character acted was DeForest Kelley.
Kelley played Dr. Leonard "Bones" McCoy, the at times dour doctor of the Enterprise. His character rarely saw any change over the decades, that was until Star Trek IV: The Voyage Home. Speaking to the press during a junket in 1986, to promote The Voyage Home, Star Trek.com published the interview in 2014, and SlashFilms brought the topic back to light a decade later.
In the junket, Kelley spoke about how he affected his character in the film, saying;
""It’s very difficult to expand or flesh a character out in a motion picture, so to speak. When we’re doing them, it takes a couple of years to get one out. If we were still doing the series, why, it would be a lot of fun to see how these characters change during the aging process. So what I tried to do in number four is kind of… not soften McCoy, but he’s become a little more attuned to Spock and he’s looking at him more or less with a bit of amusement, as opposed to becoming so irritated with him. Not that he doesn’t become irritated with Spock, but McCoy has mellowed a little bit during this timeframe. As I say, if we were doing even six 90-minutes shows a year we could expand on that, but when you shoot one motion picture every two years, it’s rather a problem because it’s very difficult to satisfy everyone. We only did 79 episodes (of TOS), but during those episodes there was bound to be one episode that perhaps everybody would like. You can’t do that with a motion picture because there’s going to be somebody who’s not going to like something about it.""
McCoy was never a softy, and even in and after The Voyage Home, it's hard to say he was a different character, though if you watch the film, you can certainly see what he's referring to. Spock had just come back from the dead, and while it was two years in between filming Star Trek's III and IV, in the movie's timeline it was far far shorter. So even though fans saw Spock alive once again to close the third film, the characters are still living with the return of Spock, and so Bones would be far more reserved with Spock.
As he did just return from the dead, essentially.