Having watched the recent Hard Talk
interview, some of the content touched on the "Shatner ego" topic and I found it fascinating. Yes, we look at Shatner today and he has played some high ego characters across the years. He does like to bask in the limelight. But I have to say that from what I've seen and read, my understanding of it is this:
During TOS, the secondary characters like Nichelle Nichols, James Doohan, George Takei, and Walter Koenig had very small roles. Doohan was the largest of the four, because he had a couple of episodes with a more prominent appearance than a usual extra. Still, the comparative screen time pales in comparison to Shatner and Nimoy. So naturally, Shatner and Nimoy would take greater precedence. A secondary actor is not going to be given nearly as much attention both on screen and off.
So, after cancellation, TOS takes off and now these secondary actors see themselves as something much more important than originally perceived. Now they feel "lessened" because they didn't get nearly as much attention (and mind you, it seemed like the stories about this increased as time went on). And if there was one or two episode scenes where changes meant the secondary actors had even less time, their perception of this is even more contentious to them... when in fact there was probably a credible production or story reason for the change. But for them, it's easy to say "well Shatner wanted it all for himself and here's a perfect example".
Doohan had a chip on his shoulder that he made well known, because he saw himself as this highly multi-talented actor who was unable to realize his potential on the show, because his role was quite small compared to Captain Kirk and Spock. But just look at what happened to him afterward. He really didn't do anything noteworthy, not one production outside of Star Trek that you could easily recall off the top of your head. Doesn't that say something... No disrespect meant for Doohan. I just don't think his talent was as compelling as he may have thought.
Everything I read of Herbert Solow's writings in "Inside Star Trek" suggested Shatner was a very professional actor who did a fine job with what he had to work with. There isn't any solid evidence at all of an ego mongering Shatner. Would you agree?