It’s also Bucky taking the 5 seconds he has of Steve not paying attention to him so he can allow himself to process all these emotions without worrying Steve. If you watch Bucky through the movies, you’ll notice he always makes sure to look like he’s 100% fine if other people are looking at him. Fighting with Steve, but smiling at their dates. Recently tortured, but walking confidently by Steve’s side. Basically a mess, but all “Let’s hear it for Captain America!” It’s a pattern, really. Even in the flashback in CATWS, you can see he looks a lot less confident when Steve isn’t looking at him than when Steve is.
Also, Seb has mentioned that researching WW2, what left the deepest impression was how quickly everybody dies. You get attached to someone only to watch their heads being blown up in front of you the next day. I’m sure this influenced how he chose to act this scene. Because you can bet by the time this scene takes place, Bucky has seen many people - hell, maybe even friends - die, and recently, he’s had to see his whole unit be killed or captured by HYDRA. This certainly plays a role here. It’s not just a general sense of “I can’t protect Steve anymore,” it’s more like “I don’t know if Steve will live till next week.” It’s very real, very immediate. It’s a concrete prediction more than a vague fear. And if Steve’s survives, there’s still the fact Bucky knows what’s like to be changed by war, and Steve will be changed by it, which Bucky certainly hates. Either way, he loses the Steve he knew, even more than he’s already lost, with the whole “Steve Rogers is suddenly a super soldier” deal.
I’d say this scene is wartime Bucky in a nutshell. He handles the entire crowd and this whole Captain America propaganda thing without hesitation, he smiles at Steve and makes sure Steve enjoys the moment instead of pulling some “I did my duty” bullshit, and only then he allows himself to be overwhelmed by the fear that comes with being able to think 48923740 worst case scenarios in two seconds. If we can trust interviews with cast and crew, this eventually becomes his role in the war, basically - he thinks fast and does his job protecting Captain America and the missions, he takes care of Steve on a personal level by shielding him from the worst of the war as much as he can, and only then, if there’s time and Steve isn’t looking, he thinks about how the war is affecting him.
But anyway, overall, this scene is about overwhelming loss of everything Bucky knows, as well as an attempt to hide this as well as he can. And I don’t think it’s a coincidence that in the 4th and 5th gifs, Seb looks a lot like comics!Bucky does when he says goodbye to his younger sister, thinking he’ll never see her again and almost breaking down in tears, but unwilling to show her he’s scared. For your reference:
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