Many of Leia’s scenes scene scattered throughout The Rise of Skywalker were brought to life using unused footage the team put together from The Force Awakens. Much of it was woven into newly-written scenes that made sense in the context of the footage available. But one flashback that needed to occur needed more imagery of Leia that the leftover footage wouldn’t be able to provide.
The scene in question would show how young Luke Skywalker (Mark Hamill) taught Leia how to use the Force. While ILM Visual Effects was able to easily de-age Hamill for his role, they couldn’t do the same with Fisher. J.J. Abrams wasn’t interested in creating a completely CGIed version of Leia for the scene, so they turned to Fisher’s daughter Billie Lourd to act as a body double. In essence, Billie “became” her mother, and took over as Fisher’s face was simply added on after filming.