Time travel is of massive importance to Avengers: Endgame, but it's not always an easy thing to explain. To help lay down the rules of the MCU's version of time travel, Tilda Swinton reprises her role as the Ancient One. She explains the damaging effects of messing with time to Bruce Banner, but apparently, this scene changed a bit over the course of filmmaking. Swinton recently revealed that she was called in to reshoot the scene a year after she originally filmed it.
In their journey to collect the Infinity Stones from across space and time, the Avengers travel to New York City during the events of The Avengers in 2012. Smart Hulk goes to the Sanctum Sanctorum and confronts the Ancient One to get the Time Stone.
It was a surprise to me, too! 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.
Specifically, the changes were a "plot thing" revolving around some key information that the Ancient One delivers to Bruce Banner, Tilda Swinton said. The Ancient One needed to explain what happens to the timeline when an Infinity Stone is removed, and how it creates a dark branched reality, while utilizing some nice visual representation with her magic.
The scene helps to set the stakes for the Avengers, who are not only risking their own lives, but the lives of an infinite amount of people across multiple realities. If they don't bring the stones back to precisely the point that they were taken, then that timeline will splinter off with some nasty consequences.
The time travel elements of the film have been a subject of debate amongst fans, as no one can really quite agree on how it all works. I would argue that the rules of time travel aren't necessarily important in this film as they are just a gateway to have some fun, but it's tough to picture how much more confusing everything would have been had they not reshot that Ancient One scene.