13. The Secret of Alice Pearce
The character of Gladys Kravitz was subject to another actor swap, but not for the same reasons as the character of Darrin. Four months before landing the role of Gladys, doctors diagnosed Alice Pearce with ovarian cancer. They treated her and she decided to keep it to herself. Then, in the second season of the show, her symptoms started to worsen. She lost a dramatic amount of weight during that season, then passed away in March of 1966.