Honestly, Sirius still never knew what to expect when he came to meet up with Regulus. He loved his brother, sure, but Reg had been hanging around those Slytherin types, the ones that were more likely to lay a trap for him than anything if Reg breathed a word of what was going on. And yea, Sirius knew he could handle it, but he still preferred to know ahead of time if is brother were planning on stabbing him in the back.
So Sirius had arrived early, far too fucking early, only to find out Reg was arriving early. But the older Black slipped back, to the loo, and came out, sticking to the wall over to the side, inconspicuously watching him for a good while. It took all his self-control not to burst up laughing as Regulus went from his nervous ticks to trying to look like a proper Black.
"Hey Reg," Sirius spoke up right then, sliding down into the seat across from him. His good fifteen minutes of watching had made Sirius fairly confident he wasn't about to get attacked from behind. "You want a chocolate cone?" Sirius asked.