"Wait!" Neville wasn't used to dealing with panicky animals, that usually went to a different department, but he was very used to being around people who were incredibly scared. He hoped it'd be enough to help here. "Please, I didn't mean to..."
He quickly tucked away his wand, wanting to prove that he wasn't going to try and force Sirius back to a man or capture him somehow. He knew a few wandless spells that could stop him, thanks to his Auror training, but maybe Sirius wouldn't assume that. He thought quickly of all the things he knew of Sirius Black and pieced together that he had to have been from after he'd been put in Azkaban but before his death. There was no other reason for him to be afraid to be known. "My name is Neville Longbottom." He did his best to keep his voice cool, made sure his hands could be seen but didn't hold them up. He didn't realize he was doing it, but he'd also slouched in his chair, tucked in on himself a little bit to look smaller, less threatening; something he'd done a lot as a kid but wasn't so much in the habit of anymore.
"I'm a friend of Harry's. My parents were in the Order. I know you didn't hurt anyone." It wasn't a lot of information, but it was the important parts, he could elaborate later, when Sirius had calmed down, if he needed to.