Warning: this article will contain spoilers for Avengers: Endgame. It turns out the Ancient One's return in Avengers: Endgame was a bit of a complicated process, but one that was extremely crucial to the end product. Tilda Swinton originated the role in 2016's Doctor Strange and, given how her journey concluded in that movie, it seemed like her time in the Marvel Cinematic Universe was over. However, as those who have seen Marvel's latest can attest to, that isn't the case. Now, the actress has explained a bit more about her return, revealing that reshoots helped make her character even more important to the movie's overall plot.

Tilda Swinton was recently making the press rounds promoting The Souvenir. During a recent interview, she was asked about making her surprise return to the MCU. Swinton, as it happens, was just as surprised as many fans. She also reveals that, initially, her work was completed in just one day. But things changed later on in the process. Here's what Swinton had to say about it.

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"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."

When pressed for more info, Tilda Swinton stated it was a "plot thing" relating to the way in which the movie deals with time travel. As we know, in the movie, the Avengers travel back to New York in 2012. Hulk is tasked with getting the Time Stone from the Ancient One, which proves to be rather complicated. At this point, the Ancient One reveals the dangers of this "time heist," and she explains the possible problems that can come of their actions. Specifically, if the stones are not put back, the Avengers could create splintered, alternate realities, in which, horrible things happen.

Apparently, these reshoots added a lot of the complicated time travel stuff into this crucial scene. Tilda Swinton said it was "all about that very, very important." One has to wonder what the character's original scene looked like and how the rules of time travel changed throughout the movie's development. Avengers: Endgame goes out of its way to make it clear time travel in their universe doesn't work anything like it does in Back to the Future. Overall, Swinton reveals the process was a treat.

"That was amazing, and a whole other world of filmmaking. But, boy, was it worth it. They tweaked it into something extraordinary."

The complicated rules of time travel in Avengers: Endgame, and the end result of the actions of The Avengers, have raised a great many questions about the MCU moving forward. While Tilda Swinton may not have those answers for us, she certainly had a big part in creating those questions in the first place. This news was first reported by IndieWire.