Tilda Swinton's The Ancient One was always a part of Avengers: Endgame, it's just that the character's role changed between principal photography and reshoots. In a recent interview with IndieWire, Swinton revealed that though her one scene only took a day to film, she was asked to return to redo it because of changes to the script. Those changes, of course, having to do with the time-traveling logic she explained in the film.
“It was a surprise to me, too!” Swinton said. “And it was a pleasure. But what was a real surprise was, we shot it one summer day, and then over a year later, I went back to reshoot it because a couple of lines had been changed, a couple of plot points had been changed. And there was a tweaking of my costume.”
While the actor apparently didn't say the exact changes, it's alluded through the report that it was because of a "plot thing" that tied into the information the Ancient One gave Bruce Banner and the Hulk (Mark Ruffalo) on the roof of the Sanctum Sanctorum during the Battle of New York in 2012. In the final version, the (at the time) Sorcerer Supreme mentioned to Banner that if he were to take the Time Stone and not return it, it'd cause a splintered reality that'd end up dooming those who lived in the fracture from the main timeline.
It's this suggestion alone why Banner not only ensures the Infinity Stones don't get into Thanos' hands during the final fight but that he rebuilds the time-traveling tech from the ground up afterward just so the splinter realities aren't created. Swinton apparently didn't mind having to return for reshoots, being sure to point out the process turned the fourth Avengers film into "something extraordinary."
“That was amazing, and a whole other world of filmmaking,” Swinton said. “But, boy, was it worth it. They tweaked it into something extraordinary.”
Swinton was just one of quite a few actors to reprise their roles for the massive blockbuster. Rene Russo (Frigga), Frank Grillo (Brock Rumlow), and Robert Redford (Alexander Pierce) were some of the biggest names to return, outside of Tom Hiddleston's (Loki) quick scene in 2012 New York.
Avengers: Endgame is now in theaters.