Every time Bucky stops, it’s when Steve is speaking to him. He stops his attacks to listen to Steve. It’s not that he knows Steve, because he doesn’t, but he feels something. There is something inside of him that is telling him to listen to this man, this man will help you, this man is here for you, this man loves you. To which, of course, he reacts with more violence because that’s all the Winter Soldier knows. He can’t process the emotions that are going through him because they don’t make sense. More importantly, he’s not supposed to feel them.
But they come when Steve talks. You can see them on his face and his eyes and the way he moves. There’s fear, confusion, anger, desperation and even recognition (thank you, Sebastian Stan, for bringing all that to life). And those emotions are enough to make Bucky stop and listen, even if just for a second.