If you remember, there was a Star Wars trilogy a few years ago in which the lead characters of the original trilogy were killed off one by one- Han Solo in The Force Awakens, Luke Skywalker in The Last Jedi, and then there were plans to center the third film around General Leia Organa Solo. But then Carrie Fisher died in December of 2016, after completing The Last Jedi. The third film had to be re-written to work around Fisher's absence.
But what happens when a star dies during filming? It happens, and studios take out insurance on their main cast, but how the production handles that in regards to the completed movie depends on how much of the role was completed, how important the character is to the story, and the whims of the producers. Different movies took different tactics, with varying levels of tastefulness. Plan 9 From Outer Space had no budget, so when Bela Lugosi, the only known star in the movie, died during production, a much taller stand-in took the role and held his cape over most of his face for his scenes. Other productions, both movies and TV series, put a bit more thought into how to handle the death of a star, as you will see in this compilation from Weird History.