29 actors who were replaced by other stars to play the same character

There are numerous reasons why some actors have to be replaced.

Some stars step in to play the same character after scheduling conflicts arise and the original actor has to leave. Sometimes the celebrity doesn't agree with the script and leaves of their own accord. Other times some set drama results in an actor being fired, and new character is introduced. 

We've compiled 29 instances of actors stepping in for each other for various TV and film projects. See how many you knew. 

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Eric Forman's older sister Laurie was played by Lisa Robin Kelly for most of her time on "That '70s Show," but she was replaced with Christina Moore on season six.

Kelly was introduced as Laurie during season one of "That '70s Show," but abruptly left the show during season three and was written out by having Laurie go off to beauty school. Kelly briefly returned on season five but was replaced by Moore for a couple episodes of season six. 

In a 2012 interview, Kelly told ABC News that she left because of alcohol abuse brought on by the loss of a child.  

"I had lost a baby," she said. "As a result of that I lost it. I lost everything and I was abusing alcohol. With 'That '70s Show,' I was guilty of a drinking problem. And I ran." 

"Riverdale's" Reggie Mantle was played by Ross Butler on season one and Charles Melton on season two.

Butler had to leave his role on The CW show due to scheduling conflicts with "13 Reasons Why." 

Melton told INSIDER that he didn't focus on Butler's portrayal of the character on the first season because he was told not to. 

"I went in during the audition process, it was like the second audition, and I had a session with Roberto," Melton said. "I watched the show to kind of get a feel of it, and Roberto [Aguirre-Sacasa] was like, 'Don't watch anything. Don't watch what Ross [Butler] did. What you're doing is completely different.' He was steering me to not throw myself off by trying to emulate somebody else."

In October, Olivia Colman was announced as Claire Foy's replacement on "The Crown."

Foy has played Queen Elizabeth for the first two seasons of the Netflix drama, but because the third and fourth seasons are set to portray the middle-aged Queen, a casting change was needed.