WARNING: The following article contains major spoilers for Avengers: Endgame, in theaters now.

Directors Joe and Anthony Russo delivered the Marvel Cinematic Universe's most epic adventure yet in Avengers: Endgame, which sends the heroes not only into space, but through time, returning them to some of the franchise's most important moments and locations in search of the Infinity Stones. Time travel comes with its own twists, turns and potential pitfalls, which made the plot device a key point of discussion during the filmmakers' appearance on Tuesday at the Smithsonian Institute in Washington, D.C.

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Anthony Russo emphasized that it's important to keep in mind that time travel doesn't actually exist. As such, the filmmakers were able to craft rules that best served the story, rather than be restricted by commonly accepts tropes. Joe Russo noted that Back to the Future informed thepublic's views of time travel and, although the filmmakers are fans, they didn't want the MCU to be beholden to that 1985 film.

That especially became apparent when three separate test screenings revealed audiences were confused by Endgame's use of time travel in comparison to Back to the Future. That resulted in the addition during reshoots of multiple jokes and explanations, including a scene in which Ant-Man and War Machine list popular time-travel films, which are summarily dismissed by Bruce Banner as he broke down the rules.

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Joe Russo pointed to two explanations of time travel in the MCU, one from Banner and the other from the Ancient One. Every action the heroes commit while in the past would result in the formation of a divergent timeline, or alternate reality, as a sort of butterfly effect. With that in mind, the Ancient One warns Banner to return the Infinity Stones to their proper places in the main timeline to ensure the trillions of lives in each of the divergent realities would be spared from the Avengers' tampering and continue to exist.

At the end of the film, with Captain America deciding to travel back to the 1940s, to a point when the prime version of himself had been frozen, and reunite with Peggy Carter, he  created a divergent timeline in which the two marry and grow old together. The question remains how Steve Rogers was able to return to the main timeline decades later, and with an intact vibranium shield to give to Sam Wilson. For Anthony Russo, the concept of the divergent timelines gave the film, and the cinematic universe, multiple layers that he particularly enjoyed.

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Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos.