Spoilers ahead for Avengers: Endgame.
Captain America and Bucky Barnes are a favorite duo in the MCU, with their friendship being vital for each character's arcs. Seeing as how their friendship is so strong, some fans have been wondering what would happen if Steve and Bucky were the ones to find the Soul Stone. The specific rules of the stone require a sacrifice and, according to Sebastian Stan, it's one both characters would be willing to make.
In the quest for the Infinity Stones, no stone costs as much as the Soul Stone. Found on the planet Vormir, the stone requires an everlasting exchange from the one looking to claim it. As the Red Skull says, it's a soul for a soul. In Avengers: Infinity War, Thanos killed his daughter Gamora, and in Avengers: Endgame, Black Widow makes the sacrifice to save Hawkeye.
Basically, shit goes down when characters go to Vormir, so how would the MCU's biggest bromance handle the cost of the Soul Stone? While in attendance at Italy’s Jus In Bello convention (via ComicBook.com), Sebastian Stan was asked which of the two characters would take the plunge off the cliff to spare the other. Stan sounded fascinated by the question and this was his answer:
Wow. [Steve] would probably be like, ‘I got this.’ And [Bucky] would have been like, ‘No, I should be doing this.' No, I think it would have been a fight, and I probably would have tried to — I should be the one, after everything, to take the leap, so to speak. But knowing him, he probably wouldn’t let me. But I don’t know, that’s a crazy question. Jesus. And then the Red Skull was there.
That sounds about right. Neither Steve or Bucky would be okay with the other killing themselves for the stone and they would definitely fight each other for that "right." It's exactly what happened with Clint and Natasha in Endgame. In fact, you could probably take almost any two heroes in the MCU, put them on Vormir, and they would fight each other to make the sacrifice.
Plus, Sebastian Stan brings up the very good point that Red Skull adds an interesting wrinkle. How would Cap and Bucky react to seeing their arch-nemesis alive as a Ring Wraith on Vormir? Cap definitely had to meet Red Skull to return the Soul Stone at the end of Endgame, so hopefully one day we get to see how that interaction plays out.