It wasn't just that the sudden appearance - and subsequent interjection - of Stark's Avox had caught Steve by surprise; it was that it demanded a recalibration, a scramble on Steve's part to reevaluate all of the things he had assumed to be true of Stark up until this point. Because Stark's Avox could speak, and it was Stark himself who had built him the tool necessary to make that happen. People in the Capitol tended to treat Avoxes like furniture, refusing to grant them anything remotely resembling personhood. At best, they were servants, but at worst, they were objects, unacknowledged until they had made a misstep, and then the punishment was normally swift and quite possibly brutal.
If Tony Stark had given his Avox back the ability to speak - and furthermore, if he clearly allowed the man to speak to him with familiarity - hell, with something that even resembled rebuke - then...
Well. That right there was enough to raise Stark more than a few notches in Steve's estimation. It was enough to reassure him that however reluctant Stark seemed to actually commit to revolutionary action, he could probably be trusted. Even if he decided he didn't want a role in Steve's plans, he probably wouldn't turn on him. Not if he was willing to go out on this kind of limb for his Avox.
The fact that the man - and Steve really needed a name for him - took up a position just off Stark's shoulder also spoke volumes. There was trust there - perhaps even friendship? - and that warranted further thinking.
"No, I - of course," Steve said. The near robotic timbre of the Avox's voice was odd, decidedly unnatural, but his words were clear and crisp. Whatever it was that Stark had designed for him, it was clearly a work of art. "I wouldn't, I'd never."
Steve ran a hand through his hair, his gaze switching back to Stark. "This is incredible," he said, and there was an appropriate amount of respect in his voice. "I didn't think - I didn't realize this was even possible." To give an Avox back their voice - that was truly a wonder. He shifted his attention back to the man behind Stark, then after a moment's hesitation extended a hand. "I'm Steve," he said, because now that it was apparent he could properly acknowledge the man, it seemed unspeakably rude not to offer at least a basic introduction. "Pleasure to speak with you."
He grinned then, uncomplicatedly happy for once, but then again, it wasn't every day you got to witness a terrible wrong made a little bit better. Not fixed, not completely, but it was a first step, wasn't it? And that certainly counted for something in Steve's book.