Star Trek VI: The Undiscovered Country was the last film in the Star Trek franchise that included all members of Star Trek: The Original Series. It was the send-off for all the characters, the last time we would see most of them onscreen. And, for the most part, it was a solid goodbye. The ending was especially poignant and left fans wanting more adventures. Apparently, at least one actor was left with wanting more for his character to do in the final movie.
Walter Koenig, who portrayed Ensign Pavel Chekov beginning with season two of The Original Series, didn't get much screentime either on the series itself or during any of the movies, although he did have more quality time during Star Trek IV: The Voyage Home. But for the actor, Star Trek VI was a painful reminder that Ensign Chekov was only a secondary character as the film didn't do much to address the lives of those other than Captain Kirk , Mr. Spock , and Dr. Leonard "Bones" McCoy . [via Screenrant]
"I found [Star Trek VI's] script to be so totally devoid of any individuality for the supporting characters. It was as if you could literally have taken one long speech and taken a scissor to it, cut it into pieces, and handed it to us. For me, it was not a wrap-up at all... We were there as expository vehicles, and that alone, and that was really painful."Walter Koenig
Kirk, Spock, and Bones were always the main characters for The Original Series so it's not surprising that the writers didn't spend too much time delving into the history or future of the supporting characters. The way the series worked was to focus on those three characters, and the follow-up movies proved to be just as successful using that formula. So obviously the team behind the final movie in the series saw no reason to change what was working...even though it made Koenig (and probably the other actors) unhappy.