I've always wondered what precisely about Shatner caused such a deep grudge in cast members like George Takei, James Doohan and Nichelle Nichols. That at least gives a glimpse of what's at the core of it.
I recall when Star Trek V was announced, and a Trek-enthusiast friend told me it would be directed by Shatner instead of Nimoy as the previous 2 were, and added contemptuously: "It'll probably be a two-hour close-up of him."
I always loved Shatner as Kirk and in other roles. But I can see as a supporting actor who was struggling for just a few lines of visibility, how Shatner squeezing you out to just more emphasize his already starring role, could be both offputting and threatening.
Takei is just ponderous and has been for years. The last time he did anything enjoyable was as the token gay guy on "Howard Stern" and even that became unlistenable as demanded to be taken ever more seriously with each appearance.
