As the Winter Soldier, Bucky's emotions were a source of vulnerability; if his handlers knew how he felt, it was easier to hurt him. I'd like to see this translating into post-Hydra aftermath where Bucky never, ever lets on when something is bothering him. That is, until the stimulus finally overwhelms him, the easy smile drops off his face, and he bursts into tears.

I'm picturing this reaction as being completely unpredictable from the perspective of Bucky's friends. Bucky seems fine until the exact moment he doesn't; you never know what's setting him off until it's too late. So this is difficult for Bucky, who feels like a crazy person who can't reign in his stupid emotions, and for Steve, who's gradually forced to accept he has to walk on eggshells around the person he should be most at ease with.

+++ for SUPER INSECURE Bucky, whose trauma minefield includes a mess of sensitive spots and fragile feelings

+ a lot of trash gets discovered this way, when Bucky starts crying during sex or when Steve is stroking his hair or kissing him. so Steve is constantly crashing into awareness of what happened during Bucky's rapes, but only once he's already upset Bucky to the point of tears. and it's really unsafe and unhealthy that Bucky won't stop him before that point

+ Steve is the one who catches the brunt of this, and it wounds his own self-confidence too. He's gently ribbing Bucky about something that happened, and Bucky's laughing and playing along, it's all cool, until Bucky starts crying, and that's when it comes out he feels so hurt Steve is making fun of him; all sorts of things that make Steve feel like the asshole

+ Bucky is a fidgety but silent crier, because what's sadder than that image? also it leaves open the possibility of Steve fucking Bucky from behind, only to discover once he finishes that Bucky has been crying for like ten minutes and the pillowcase is soaked