Well based on all the things I've read and heard here's what I consider to be the true story. Take it or leave it as you will.
1. Shatner clearly pissed off every other actor on the show, except De Kelley who I think had been around hollywood long enough that he knew all the bs and politics and was just happy to have a job. With Nimoy it was both wanting to be the alpha dog and with the others it was having to lose screen time, lines and generally cowtow to Shatner, despite the fact they were hired as SUPPORTING CAST MEMBERS. Was it nice for Shatner to treat them in ways he did....no, but was he the star and they weren't....yes. Happens all the time in life where your superiors at work are jerks, it sucks, but you know what learn to deal.
2. Don't think it left any lasting scars on the cast after ST was canned, it was just another short lived TV show going to the scrap heap and they were more worried about their next job as opposed to harboring resentment to this guy they worked with for a few years.
3. Star Trek comes back to life. So they have to work with Shatner more and put up with more of his behaviors, but noone seems willing to say anything probably because they were afraid of losing their jobs and being written out of any future ST films. Also I believe that in the time frame between the end of TOS and the start of making TMP, when Star Trek was extremely popular even there was no actual product that the supporting cast members began to retroactively think of Star Trek as an ensemble show, like Seinfeld, where all the characters got major parts in most episodes, when it never was an ensemble show to begin with. They felt this way robably because they were always appearing in public together and also because the media was saying how the show was so groundbreaking because it had an African American female actress, a Japanese American actor a guy pretending to be Russian and it broke a lot of social barriers. So IMHO their own importance to ST grew by the time TMP came around and I think they expected bigger roles. Which of course didn't happen for the most part.
4. So they're relegated to having not much to do and once again being second fiddle to the Shatner-Nimoy-Kelley trio of logic vs. emotion and Shatner picks right up where he left off making sure he gets his first, often at the expense of the supporting cast, which peaks with him in TFF, only now he's doing it with something that is known worldwide instead of a little known 60's TV so the resentment grows with the popularity. But again what are they going to do? If they speak out they risk getting canned and noone was lining up to make "THE ADVENTURES OF CAPTAIN SULU" or anything so they keep their mouths shut to keep a steady paycheck, which I can't blame them for.
5. In fairness though I don't think you can really jam in 7 important character arcs into a 2 hour film. So you gotta go with your strongest horses and the public wasn't dying to see a film that centered around Chekov's past coming back to haunt him.
6. Then a big turning point comes....Shatner writes Star Trek Memories. I think he should be commended because he included interviews from all the actors and other people involved, except Doohan who refused to be interviewed, he didn't have to include their stories but he did. Also at the end he openly tells the story of how someone, Nichelle Nichols, finally had the balls to call out him out on how he treated others during ST. Shatner acknowledges he made mistakes and hurt people and said he was sorry but his passion carries him away. How sincere he was I don't know, but at least he had the guts to put it in print. He could have just left it all out. Then he relays how Takei and Koenig were going to talk to him about it as well in there interviews but Koenig changed his mind and he and Takei had such a great time talking that George decided not to ruin it with a confrontation. Shatner even says something to the effect how he admired Takei a lot for all he went through as a kid during WWII and thought he was a great person for how he kept working despite the odds he faced. He also says how Doohan flatly refused to be interviewed about saying Shatner would only twist the words to his advantage.
Then in one of those "damned if he does, damned if he doesn't" moments the fans put him Shatner personalls says to Doohan if you're reading this and you want to get things out there please give me a call. A lot of people said Shatner said that to make himself look good, which I think is BS. What did people want him to do say "FU Jimmy, if you can't get over things then take this book and cram it up your faux scottish ass."
Well after that you'd think things would calm down.....wrong. The Shatner-Doohan feud becomes an open secret with fans wanting to know more. Then TUC ends the original cast's run so now they don't have to worry about being fired so the comments about Shatner seem to come more and more frequently......despite the fact he seemed to address those in Star Trek memories.
Then Takei takes it to a new level in his book "To the Stars"......which is crap BTW. The cover his him in his film red uniform looking towards the heavens and it says "By George Takei.....Star Trek's Mr. Sulu" Yet only about 25% of the book actually deals with Star Trek and the rest is his rather boring personal life. In the time he does talk about Star Trek he spends a good amount of time attacking not just for his antics on the set, but also as a person. His biggest bug up his ass is how he'd convinced Harve Bennett that Sulu needed to advance as a character so there was originally a scene in TWOK where Kirk informs Sulu he is to take command of Excelsior after the Enterprise training cruise is over.
Takei gets all upset because he didn't think Shatner played the scene with any emotion because it didn't center around him....whatever that means, and it is ultimately cut out of the film, which he seems to blame on Shatner but can't really prove it was his doing. So he's upset his fictional character didn't get a fictional promotion to command a fictional ship. But it would have been stupid anyway to put it in TWOK because in TSFS he would have immediately had to give it up to help save Spock, or take command and try to chase down the stolen Enterprise and looked like a fool when the Excelsior's warp drive failed. But you honestly would have thought the way he carried on you'd think that the US Navy and Paramount had an agreement where Takei would become commander of a real warship and Shatner ruined it. Takei also relates how he was horrified when Koenig told him that Shatner was to direct TFF and he believed the story was going to be how all the crew members disappear except Kirk and they're all replaced with William Shatner speaking in their voices.
So years roll by, the internet becomes bigger and bigger and ST fans learn more about Shatner's ego and his co-stars feelings, especially Doohan. Supposedly they reconcile before shortly before Doohan dies, some believe it, some don't. Now I truly believe Doohan did really hate William Shatner, but I don't believe it was because Shatner was always screwing him out of screen time. If you watch TOS Scotty actually got quite a bit of screen time and was a deeper character than Nichols, Takei or Koenig....In fact I would be willing to be his screen time wasn't that much behind Kelley. I think Doohan was a man who'd grown up doing things like fighting in wars, killing Germans, getting shot himself, performing airplane stunts some called insane and in general was a pretty bad ass man's man and I think he just resented Shatner who had known pretty much nothing but acting since college acting like he was some macho stud and throwing his weight around when he'd never faced fear and death like Doohan had and he couldn't respect a man who acted like that with no concept of what true respect entailed. Unfortunately I think he let it poison his feelings too much and he honestly went to his grave still resenting Shatner which is too bad.
As for the others. Nimoy and Shatner realized that it was to their mutual advantage to be friends instead of antagonists, Kelley I don't think really gave a shit one way or the other. Nichols and Koenig seem to take their shots from time to time, but don't seem to revel in it.
But Takei still seems to want to keep this thing, whatever you call it going. And there are two possible reasons IMHO 1. He really resents Shatner and is going to be like Doohan and keep letting people know until one of them dies or 2. He realizes it brings him publicity to keep this thing alive. I tend to believe the latter.
The problems I have with this whole thing are these:
1. Despite whatever Shatner may or may not have done to them, the fact their lives are much better and more prosperous because of him. ST survived because of 2 big reasons: Many of the stories were compelling and entertaining and the other reason is the performances of the main actors, especially the whole logic with Spock vs. McCoy with his heated emotions and Kirk trying to balance the two. If Kirk had been played by an actor who did a lousy job the whole show might not have worked and once it died that would have been the end. But Shatner as Kirk was popular and helped the franchise come back to life and they ALL got much more money, fame, and opportunities as a result. You'd think they could say "OK Shatner was a jerk, but it was a price a paid and what we got far outweighs what we dealt with."
2. Quite frankly I think the supporting cast were chickenshits the way they always tried this passive/aggressive approach to informing people about Shatner while their jobs still depended on it. Either confront the man and get it out and if it still doesn't improve then quit. TFF would have been a perfect time to make a statement. I'm pretty sure by then all of the supporting cast were financially secure for life. If they hated Shatner so much they should have marched into Paramount's office and said "We've had enough, we're not going to be in a film he's directing too and that's that" and marched out. THAT would have been a statement. Instead they kept cashing the checks and pissing and moaning through the media about what Bill did or had done. Most of us have to deal with jerk bosses or co workers, but we aren't fortunate enough to have that same jerk help bring us major wealth and priviledge.
And my final one, directed at Takei himself, is he's totally been a whiny hypocrite about it for almost 25 years now. First he seemed to forgive Shatner in ST memories when Bill gave him a chance to get it out man to man. Then he waited until the run of TOS cast was over until he let loose with the big guns, because he didn't want to lose a paycheck. Then in the book he personally attacks Shatner saying something along the lines of while filming TUC he watched Bill and thought how sad it was the determined young actor he knew on TOS had become such a buffoon and diva.............Then years later Takei goes and does the EXACT same thing. I'm not going to get into all the social commentary and that stuff but Takei IMHO very dignified in his actions in real life throughout ST's run. But when he realized the show was over and there spotlite was off he had no problem becoming a guy who acted in over the top and ridiculous ways for cheap laughs to keep himself relevant. I personally think it's embarrassing how he acts on talk shows or in cameo roles, and it has NOTHING to do with his personal life, he just seems like a buffoon. Shatner was always known as silly and ridiculous in some ways, and it just seemed to get worse as he got older. But he also still showed he could act in Boston Legal. All I see Takei doing these days in his appearances is either saying "Oh my" or some other lowbrow humor or trashing on Shatner.
So no I don't think it's a feud. I think it's a guy who sees that he can stay in the spotlite some by ripping on a guy he hasn't had to work with in 25 years and personally I think it's a lame and stupid thing to do instead of just letting it go and living out whatever years you have left without trying to profit at someone else's expense.