Botany Bay wrote:
When Gene Roddenberry was asked about this non-part, he allegedly said : "I only hired her because I wanted to score with her."
He must have wanted that score pretty bad to promise her third lead
That bit of apocrypha is from Inside Star Trek, and it's just one of the many anecdotes related, but which are hard to prove.
As to the contracts and actors who were not brought back, it's inaccurate to say that any of the actors were "fired". Second pilots are rare, and contracts (at least in the past) would not have included them (and there are practical reasons an actor would not wish to be tied to such a requirement). When the first pilot didn't sell, all the contracts were effectively terminated, which is why they could not compel Hunter to return and had to sign a new contract with Nimoy. The studio had no option on the actors and likely would have had to negotiate new contracts had they wanted any of them to continue. Since NBC didn't like most of the cast, this wasn't going to happen.