Imagine Bucky being really good at talking to people he doesn't know because the Red Room anticipated him needing to infiltrate places but when it comes to talking to the people he loves - Steve - he just. Can't.
Steve can’t understand it. he and bucky used to tell each other everything, they were so close they were like one person. Bucky still has his easy, natural charm talking to the press, at SHIELD debriefs, hell, he even seems more comfortable talking to bruce and thor than he does with Steve. At first steve thinks it’s because bucky still feels guilty about having tried to kill him, but he doesn’t act ashamed.
Then it strikes steve that whenever they’ve tried to talk, it’s been like they used to, like friends talk, and that’s when bucky clams up. With the press he’s all glitz and glam, charm without substance. so the next time steve winds up in the kitchen at the same time as bucky, he makes meaningless small talk, things that he might say to anybody. it’s not much better, still a little stilted, but bucky seems to be able to reply to him like this without tripping over his own tongue, and steve can see the gratitude in his eyes. It’s progress, he thinks, and they’ll get there.