The TOS movies, at least for a while, made an attempt to address some of this in a realistic manner, acknowledging, of course, that they had to keep the cast together. But I think they eventually lost their way.
In TMP, we see that some people -- like Kirk, Spock, and McCoy -- are no longer aboard the Enterprise, having either been promoted or left Starfleet. Of the remaining crew, we've seen promotions in rank and Chekov is now Chief of Security. Scotty is still Chief Engineer, but we know from his character that the Enteprise engine room is where he wants to be, and I think it's realistic that Starfleet would have had him oversee the refit of the ship. Only Uhura and Sulu are really hard to explain and, even then, haven't they moved up in rank?
By the time of TWOK, we again see some realistic development. Kirk is an admiral at Starfleet. Spock is in command of the Enterprise. Chekov is first officer of the Reliant. We don't really know what McCoy, Uhura and Sulu are doing, but it seems clear that at least McCoy and Sulu are not stationed on the Enterprise. Only Scotty is still in his familiar post which, as above, makes sense for him.
TSFS and TVH are special cases because the crew are not working for Starfleet. They are fugitives acting on their own, banding together with their closest friends and comrades. Makes perfect sense for them to be together.
TFF is where it really starts to fall apart, in my estimation. It was obvious that they wanted the 'status quo' for that film. Now, Kirk is back in command as a result of the events of TVH. That I can accept. That's what he always wanted and Stafleet gave it to him. I can also accept that Spock and McCoy would be willing to be at his side and that Scotty would still want to be in the Enterprise's engine room. But Sulu, Chekov, and Uhura begin to stretch the bounds of credibility. While I can understand their loyalty to Kirk and his wanting them as part of his crew, would they really stand still for that long? Sulu and Uhura are at the same stations in the same positions they've held for, what, 30 years at that point? And Chekov, having been promoted to Chief of Security and again to First Officer is now... navigator?
TUC attempted to undo some of that. Sulu now has his own ship. The rest of the crew end up back together on the Enterprise, but it's clear they've been pulled together specially for this mission, rather than being assigned their normally. Uhura is supposed to be off lecturing at the Academy. Spock is involved in diplomatic matters. Chekov seems to have gone back to a more security-oriented role. Even Scotty, long attached to his engine room, is speaking enthusiastically of his impending leisure time.
All in all, I think the TOS films, with the exception of TFF, did a good job of plausibly keeping these characters together.
Now, TNG? Can't even begin to tell you...
