In the show Dr. Who, the original role of Dr. Who was played by actor William Hartnell. Hartnell had to leave the show due to arteriosclerosis. His health was quickly deteriorating and it was so hard for him to keep up with the intense filming schedule, so he had to plan his leave. Luckily, the producers and writers had an amazing plan. They were going to have the Dr. get himself a new body!
Dr. Who was a cult classic. The show was making serious money and they were not about to cut this run short. Hartnell wanted the show to go on as well and he had the idea that Patrick Troughton could become the new Dr. Who.
Robert Downey Jr. on Ally McBeal
On the show Ally McBeal, Robert Downey Jr. played Larry Paul, who was the love interest of the title character, Ally McBeal. Ally had a multitude of different love interests on her show, but the team brought Larry in as the last one. The plan was for her to marry him. That was a great idea since he was a big fan favorite - both character and actor wise.
Robert Downey Jr. got the role of Larry Paul shortly after getting out of jail. He was in and out of jail due to his addiction to drugs but was hopeful that he would be able to stay clean now that he had a new role to focus on. Sadly, that was not the case, and he got fired from the show after his inability to stay clean.
Paz De La Huerta on Boardwalk Empire
This is probably one of the strangest reasons we have seen yet for being fired from a show. Actress Paz De La Huerta was an actor on the show Boardwalk Empire. During the filming of one episode, naked, she had a tampon string hanging out between her legs. She was told to hide the string, or get rid of it - at which point she pulled the tampon out and dropped it on the floor, beckoning a personal assistant to pick it up.
Paz De La Huerta had gotten into trouble previously, and she was even arrested and charged with assault for throwing a glass and cutting a fellow actresses' leg.
Yvette Nicole on Community
Yvette Nicole decided to leave her role as Shirley, on Community, right before season 6. She was on the show since the beginning, in 2009, so it was a big surprise when she asked to be written out. Sadly, her dad was very sick and she wanted to leave the show so she could go and spend time with him.
She had said that her father needed to have round-the-clock medical care and that he really needed her. She could not bear to be away from him any longer. She did admit that it was a very difficult decision. Luckily, she was able to return to the show for the series finale.
Lecy Goranson on Roseanne
In the instance of the show Roseanne, they had two actresses play the role of Becky. Lecy Goranson played Becky for 8 seasons, and then Sarah Chalke came in and replaced her after that. Becky one was, of course, the main character, so fans were left wondering why a brand new actress was playing the role without any warning.
Goranson decided to leave Roseanne to attend college - and the show was left with a difficult decision. They decided to keep her character on and just replace her with another actress. Strange move, but probably a good idea. Goranson did end up coming back on the show during season 8.