[The following contains spoilers for Star Wars: The Rise Of Skywalker, but you probably saw it already, right?]
One of the most… surprising things about Star Wars: The Rise Of Skywalker was the high level of screen time granted to Carrie Fisher’s Leia despite the fact that Fisher died long before filming started and all of her scenes were Frankenstein’d together using leftover footage from Star Wars: The Force Awakens. Depending on how well you stomached it, Leia’s scenes landed somewhere between being a bittersweet tribute to the character and being a jarring distraction that reduced the character to a depressing puppet who was brought to life exclusively to save Disney the trouble of having to write around Fisher’s death, but that’s the what we got and there’s no way for anyone to go back and change bad things in a Star Wars movie.
Anyway, the one and only legitimately new thing we got from Leia in The Rise Of Skywalker—not counting the lightsaber that nobody ever thought to mention—was a brief flashback sequence revealing that Leia had trained in the ways of the Jedi with Luke at some point after Return Of The Jedi (another thing nobody thought to mention). If the scenes with present-day Leia stood out because they were so obviously structured around an actor who was not there, the flashback sequence stood out because it couldn’t possibly have been Carrie Fisher playing the younger Leia. Now, thanks to an interview that Industrial Light And Magic’s Patrick Tubach gave to Yahoo! Entertainment, we know who really was swinging that new lightsaber at young Luke (who was played by a digitally de-aged Mark Hamill).
This comes from Entertainment Weekly, which says Fisher’s daughter Billie Lourd was the one who played young Leia, with Fisher’s face being recreated from Return Of The Jedi-era archival footage. Filming this scene, despite how short it was, was apparently a very emotional thing for the crew and Lourd, with Tubach saying it was “something that nobody took lightly. Director J.J. Abrams was apparently very insistent that nobody would play Leia other than Carrie Fisher, but he evidently wanted this scene in the movie bad enough that he was willing to go back on that—but only if she was played by Lourd, and only if Lourd was willing to do it.
It seems like a lot of trouble to go through for a short scene that arguably didn’t work or make sense in the context of the movie, but that’s (arguably) just The Rise Of Skywalker in a nutshell.