Sirius waited out in the hall while Regulus had gone instead to talk to McGonagall. He glanced around, making sure nobody was around to see and quickly transformed into a large black dog. He would be able to hear better without leaning against the door and risk being found eavesdropping. Sat quietly, not moving until he heard their voices approaching the door and he quickly transformed back, extremely glad that Animagus transformations allowed him to keep his clothes when he changed back.
He frowned a bit once the door opened, a little irritated that McGonagall didn't really even try to make him stay. Sirius finally got proof that his brother might not be the idiot he always thought he was and now he wanted to go off and get himself killed. Not that Sirius would forget anything that had gone on between them. That Regulus had chosen their father and mother over leaving with him or what side he had chosen. He was afraid of what Voldemort would have become and of the help he had been giving him. He couldn't forget that nothing would be any different if Voldemort hadn't used Kreacher the way he had.
"All settled then?" he asked, brushing off his denims and turning his torch off and on again. Sirius was once again disappointed in Regulus, but at least it was for a slightly better reason this time.