WARNING: Major spoilers for Avengers: Endgame.
Natalie Portman returns in Avengers: Endgame as Jane Foster, but she didn't really film anything for the movie. One of the major early additions to the Marvel Cinematic Universe, it's been over five years since Portman last appeared as the world-leading astrophysicist. She debuted in 2011's Thor, but she split from the franchise after some disagreements on Thor: The Dark World - namely that Patty Jenkins (future director of Wonder Woman) left the project due to creative differences.
After the apparent falling out between Portman and Marvel Studios, Portman revealed that she was done with the MCU - as far as she knew - despite a great potential future ahead. This is what made her sudden appearance at Avengers: Endgame's premiere all the more surprising. As audiences around the world have since learned, Portman is among the many MCU veterans who make surprise returns in the 22nd MCU film.
Her appearance in Avengers: Endgame didn't require her to do any filming, however. During an interview with EW, directors Anthony and Joe Russo confirmed that Portman's return was done using cut footage from Thor: The Dark World. This doesn't mean that Portman was completely absent from making this return possible, as the directing duo revealed that she did come in to do some voiceover work for a small scene of her talking in the distance.
The return of Portman's Jane Foster comes during Thor and Rocket's time travel mission to collect the Aether/Reality Stone. They have to travel to Asgard during the events of Thor: The Dark World in 2013, where Jane is being protected after she becomes the host for the liquid-like power. Thor can't summon the courage to face her though, leaving Rocket to extract the Aether out of her body. The only time we see Portman onscreen is right after she wakes up and is about to be poked by Rocket.
Even though the return of Portman was done through old footage and voiceover, it has given fans a renewed hope that she could still come back for a larger role in the future. Just last year, Portman told Screen Rant that she was open to returning to the MCU in the future, and as Phase 4 and beyond of the MCU start to take shape, this small and easy return in Avengers: Endgame could be the start of Marvel Studios trying to bring her back completely. Thor is off with the Guardians of the Galaxy now, and Valkyrie is ruling over New Asgard, but maybe Jane could take her place as the new Thor in the future. There's plenty of possibilities with new movies and Disney+, so it'll just depend on whether or not all parties involved can figure out an exciting storyline to warrant Jane Foster being brought back into the larger MCU narrative.
Source: EW
