The presence of the Guardians of the Galaxy in Avengers: Infinity War helped make up the emotional core of the film. Their role on Titan, not to mention how crucial they were in rescuing Thor and setting him on the path to Stormbreaker, helped the heroes put up a fight against Thanos.

But surprisingly, according to GotG director James Gunn, their role in Infinity War wasn't supposed to be nearly as substantial. Initially, Gamora and Nebula were just going to cameo. Thankfully, that decision was changed because in the bigger picture of how Endgame unfolds, the daughters of Thanos simply needed to have major roles in the previous movie to set up key dominoes.

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Gamora's arc in Infinity War couldn't be covered in a cameo. The first film had to set up the stakes and enable us to understand Thanos' single-minded  dedication to his warped mission -- to the point that he'd kill his favorite adopted daughter on Vormir. That way, when the past version of Gamora appeared in Endgame, we'd be able to empathize with the emotions Peter Quill and the Guardians were experiencing.

Also, when Earth's Mightiest Heroes teamed up against Star Lord on Titan, Gamora's death is what spurred him on. His mistake in this battle is one of the things that allowed the Mad Titan to get the Time Stone, setting the movie's final showdown in motion. In other words, the assassin added emotive layers to the Guardians, as well as to Thanos' mission to course-correct a broken universe -- whether others agree with his methods or not.

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As for Nebula, it's easy to see why Gunn's ending in GotG Vol. 2 led to her having a bigger role in Infinity War. She left the Guardians sequel on a note of reconciliation with her sister, but she still went off to kill Thanos alone. In Infinity War, she'd been imprisoned and tortured by Thanos, adding more layers to the family dynamic of Thanos, Gamora and Nebula, and affirming the love Gamora and Nebula have for each other.

Not to mention it sets Nebula on a heroic path as the death of her sister makes her want Thanos dead even more. This is why she becomes an Avenger in Endgame, completing her redemption After seeing her heroic side in two movies, the Russos are justified in having her run around with the Nano Gauntlet in the finale, as she's become more than a supporting act.

Thanos torturing Nebula as she screams.

In Endgame, this also creates tension between Nebula and the younger Gamora, as well as the Mad Titan from 2012. If Nebula and Gamora had only minor roles in Infinity War, this rapport just wouldn't feel natural and we'd be missing a fundamental element of Thanos' sacrifice. Simply put, the sisters had such important arcs, they needed to be spread across two movies to make a substantial impact.

Gamora and Nebula contributed to Thanos' purpose, his ambition to balance the cosmos and, also, viewers' understanding of just how sadistic the villain was, as seen by the horror he inflicted on Nebula when interrogating her. And, as tough as their journey was, the sisters laid the foundation for a solution to the atrocities their father committed.

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