IGN recently spent some time speaking with visual effects supervisor Roger Guyett, creature effects supervisor Neal Scanlan, and ILM visual effects supervisor Patrick Tuback on the subject of re-purposing unused footage from The Force Awakens to bring back Carrie Fisher for one final performance as Leia in The Rise of Skywalker.
Quite a bit of work was involved to accomplish this goal, especially since the Leia we would be seeing on screen in TROS would have a few differences from her appearance in the previous Sequel Trilogy films, as noted via the following comments:
Sharp-eyed fans might have noticed some differences in Leia from the other two films, including a new outfit, new hair and new jewelry. That took a lot of work as well, according to Guyett. “To make that work, there was a tremendous amount of pre-planning we had to do. We had a marvelous stand-in for Carrie, an actress who learned all of the pieces that we needed so that we could shoot the thing a little bit more organically.”
It wasn’t just changing her appearance. The VFX team had to build her into the scene. They had to match the lighting, the camera angles and the staging to make it look real. “You can’t just do a scene and then suddenly keep cutting to Carrie as a single shot and she says something. You have to build her into that scene. So we wanted her to move, we wanted camera moves. We want it to feel as though she’s right there and you’re doing it. And that’s really where it got complicated and gnarly.”