The Russo Brothers have, in many ways, served as architects for the Marvel Cinematic Universe ever since their first film, Captain America: The Winter Soldier. Joe and Anthony Russo have gone from working on the hit comedy series Community to creating four of the most well-received MCU films: Captain America: The Winter Soldier, Captain America: Civil War, Avengers: Infinity War, and the upcoming Avengers: Endgame.

Each film felt tonally different -- The Winter Soldier forced Captain America to confront the simple black-and-white morality that led him through World War II no longer applied to this modern world, and drew heavily from political dramas featuring corruption and espionage. On the other hand, Civil War forced the Avengers to face a disagreement that divided the team beyond repair, questioning if they required oversight to ensure they did the right thing, or if oversight would result in more harm than good. In Infinity War, our heroes dealt with an enemy they couldn't defeat, and the loss of life that came with the defeat.

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According to Joe Russo, the one key factor running through their four MCU films is the question of what it means to be a hero.

“In Endgame, it’s exactly what you alluded to," Russo told BoxOfficePro. "What is the definition of “hero”? All four of those films, when looked at together, ask that question and try to answer that thematic: What does it mean to be a hero and what does it mean to stand up?”

This theme is supplemented by many of the other MCU films as well. Tony Stark finds his sense of heroism in a cave with a box of scraps. Scott Lang tries to be a hero for his kid after leaving prison. Peter Parker has to learn how to be a friendly neighborhood Spider-Man. Most recently, Carol Danvers' entire arc was to understand the moral ambiguity of her actions as a Kree Soldier, only to become a hero on her own terms.

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The Russos have very meticulously challenged every prior definition of heroism in the MCU with their quartet of films. Avengers: Endgame is the final answer to their personal thesis: What does it mean to be a hero?

Directed by Anthony and Joe Russo, Avengers: Endgame stars Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Don Cheadle, Paul Rudd, Brie Larson, Karen Gillan, Danai Gurira, Benedict Wong, Jon Favreau and Bradley Cooper, with Gwyneth Paltrow and Josh Brolin. The film arrives April 26.