From what I can tell he's had a bad relationship with a number of the other cast members since the original series's filming, since they thought he was too self-centered and kept trying to minimize screentime given to the supporting cast. The feud became kind of well known among fans, because four of the actors (Walter Koenig, James Doohan, Nichelle Nicols, and Takei) often aired their dislike of Shatner at Star Trek conventions. Though for his part Shatner claims it was a one-sided feud he didn't even know about until 1993: he says he discovered the issue when he tried to interview a bunch of the old cast for his memoir, and several wouldn't talk to him, or did but had negative things to say.