Allyn Gibson wrote:
It doesn't make sense that she'd be optioned for another film when other actors were renegotiating their contracts film by film.
According to George Takei and Walter Koenig, each ST movie contract did have a clause that was more like an expression of interest rather than a watertight agreement to do a sequel for a set amount. Such a clause supposedly makes the negotiation process easier, since the parameters for proposed future salaries are already set up. But yes, each movie was renegotiated as a fresh contract, despite the media reports that would say things like "and they have agreed to three more movies".
George himself held out on ST II till the second day of principal photography. There was no contractual compulsion that he had to participate. Harve Bennett promised the promotion-to-captain scene as the final bait that Sulu wouldn't just be sitting in the helmsman's chair for the whole movie.
When I interviewed Billy Van Zandt about ST:TMP, he commented that the extras and bit part actors also had a presumption that, had TMP gone back to telemovies or a "Phase II" series, that there were jobs for them. I also read an interview with a bit actor about ST III (in "Cinefantastique") where she said that the background actors and stand-ins from ST II were pleased "to be back".