Actress Carrie Fisher to appear in Star Wars IX through the use of previously shot film footage.
Carrie Fisher, who passed away in December 2016, will appear in Disney’s third Star Wars movie, according to the Daily Mail in "Carrie Fisher's brother confirms Princess Leia WILL appear in Star Wars Episode IX…which will be released three years after her death."
Fisher's brother and her daughter have agreed to allow Disney to use footage in the third film of Fisher that was previously shot. The footage will presumably be edited in a way that makes it appear that it was always shot for the film.
When Disney purchased the rights to the franchise and announced that it would make new films a few years ago, Fisher agreed to play the same role in three of the new movies. She passed away after the second of those movies was filmed and before filming on the third began.
This is one of the things that celebrity estates often have to deal with. How the celebrity's image can be used post-death, is a complicated issue that requires competent estate planning and estate administration.
Reference: Daily Mail (April 7, 2017) "Carrie Fisher's brother confirms Princess Leia WILL appear in Star Wars Episode IX…which will be released three years after her death."