Sirius stopped a moment, having more trouble than he thought he would actually talking to his brother in person. It was different sniping at each other through those books, but after so long of not speaking, he didn't really want it to stop. He hardly thought grunting and pointing would be a good way to leading him to McGonagall though.
He lit a cigarette and put it in his mouth before he clicked on the torch he'd salvaged from an empty Muggle house and shined it at Regulus, aiming for his face so he couldn't see who he was right away. "We're going straight to the ones in charge. Do not talk to anybody along the way," he said, pausing a moment before shining the light elsewhere. "I'll kick your arse if you try anything."