Bucky is torn between pushing the issue because he's so sure now that he's looking that Steve is different, and pretending he hadn't said anything out of terror that his already tenuous grasp on reality is slipping. Surely Steve would have noticed his body changing, right?

"Steve-- just..."

He hesitates and there's something dreadful and pained about the muted look of terror behind his eyes. He's scared of losing himself, genuinely not playing a joke on Steve.

"...just go into the bathroom for me, take off the hoodie and look at yourself in the mirror. Please."