While the ending of Avengers: Endgame took some of the characters -- and fans -- by surprise after Steve opted to stay in the past and live out a life with his love, Margaret "Peggy" Carter, there was at least one person that knew of Steve's plans before he journeyed into the past with the stones and Mjolnir -- Bucky.
At the end of the film, after the group returns from saying their goodbyes to Tony, Steve readies to return the stones to their rightful timelines -- along with Mjolnir -- by re-entering the quantum realm. We see him working with Bruce, who assures everyone that it'll only take a matter of seconds for Steve to return. However, after the five seconds are up and Steve doesn't return, Sam, aka Falcon, demands Bruce figure out what went wrong while Bucky wanders off with a knowing smirk. It's an implication that he knew Steve didn't intend to return, but for the first time, the writers have confirmed it. In a new interview with The Hollywood Reporter, writers Christopher Markus and Stephen McFeely state that Steve and Bucky briefly talked about Steve's plans.
"Steve, clearly, before he stepped on that platform… told Bucky what he was going to do. Whether he knew he was going to appear on that bench, I don't think so. Why would Bucky say, 'I'm going to miss you pal,' if it was only going to be five seconds?" McFeely said.
As for the passing of the mantle, did Steve ever offer Bucky the mantle? Per Markus and McFeely, that was not a conversation that happened between the best friends. Instead, the only conversation regarding the passing of the Captain America mantle is the one we see on screen between Steve and Sam, with Bucky urging the latter to accept. Unfortunately, for some fans, this is in direct contrast to the theory that Bucky had rejected the mantle prior to Steve offering it to Sam.
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.