Sirius wasn't going to admit it to Regulus, but he was impressed. Not so much with the fact that Regulus didn't seem to have a plan at all or that he seemed to know what the hell he was doing, but he had apparently decided to make some move against both Voldemort and their mother. A little too late, really, but it was something.
"Go and talk to McGonagall and tell her whatever it is you needed to say. You won't get out of here without any help anyway." Sirius said, fully prepared to listen at the door to whatever this big secret was.