“When negotiating Kirstie Alley’s contract for Star Trek II: The Wrath of Khan (1982), Paramount Studios did not offer or include any options or clauses regarding any possible sequels. According to director Leonard Nimoy, this left Alley open to negotiate a new contract for this film, resulting in Alley’s excessive salary demands, which led to her being dropped and replaced by Robin Curtis.”
https://www.imdb.com/title/tt0088170/trivia/?ref_=tt_ql_trv
Star Ricardo Montalban was hailed as Khan......
said he wanted to do the sequel so badly he took
far less money than he usually made.
III was one of the weakest in the series.