Just days after Fran Grillo announced his return as Crossbones in Avengers 4, producer Michael Grillo has confirmed that Tilda Swinton will return as the Ancient One in Avengers 4. She initially died in "Doctor Strange," so this just continues to prove that Avengers 4 either involves time travel or flashback sequences. Screen Rant was the first to break the news, saying that Grillo made the announcement during the Austin Film Festival.

Avengers 4 has become Marvel's best-kept secret to date. Marvel Studios have worked hard to ensure viewers don't know what to expect from the blockbuster; we still don't even have a title for Avengers 4.

Despite all their precautions, there has been a constant flow of rumors that the next chapter in the Avengers would involve time travel, with some reprising their roles, most recently Crossbones and the Ancient One.

Michael Grillo says Tilda Swinton was only needed for a day

Comic Book says that Grillo made the announcement as part of a panel at the Austin Film Festival. The panel included Avengers 4 screenwriters Christopher Markus and Stephen McFeely, and Grillo who serves as executive producer. During their panel discussion, they mentioned how much of a logistical nightmare scheduling filming was because of the number of A-list actors that are part of the blockbuster. Grillo ended up spilling the beans on Swinton's return saying "she was just a 1-day availability."

It's been believed by many that the untitled Avengers 4 film will somehow involve time travel, leading the surviving Avengers to travel back in time to prevent Thano's destruction of half of the universe.

However, fans are not sure how this will be accomplished, but it will have to involve revisiting significant events where Infinity Stones were present. It's been rumored that the Avengers will travel to Kamar Taj, to meet up with the Ancient One, who was the original guardian of the Time Stone. Avengers 4 could also involve time travel within the quantum realm.

The Ancient One could have a major role in helping thwart Thanos

The Ancient One has studied the Time Stone for years, making her one of the most knowledgeable in the entire history of the MCU. She could explain how time works in the MCU and help the Avengers succeed in their quest to reverse everything Thanos has done and stop him from getting all the infinity stones.

While this news is a surprise to Marvel fans, we must remember that Swinton was only needed for one day, which means her role will not involve a major amount of screen time.

Avengers 4 is building up to be an incredible end to the first three phases of the MCU. It's being described as a trip down memory lane for Marvel fans, visiting key moments in the MCU's history.