Tina Louise was reportedly very much a diva in the day.
She insisted that she be the last credit mentioned in the opening theme song, which is why in the first season, the Professor and Mary Ann, originally considered "second-billed co-stars", were referred to as "and the rest."
Afterwards, Bob Denver personally went to the studio and asked that Johnson and Wells be added to the theme song's opening credits. When the studio at first refused saying it'd be too expensive to reshoot, Denver insisted, even saying that if Russell Johnson and Dawn Wells weren't included, he wanted his name out of the song as well. The studio caved in, and "the Professor and Mary Ann" were shoehorned in at the very end of the theme for Seasons 2 and 3. Dawn Wells said Denver never mentioned this to anyone in the cast, and she did not find out about it until years after the show ended.