Those emotions are not, in fact, kept from Sirius' face and they hit Regulus like a physical blow. He's gotten good at hiding his emotions, at compartmentalizing so that he doesn't get caught but this is... different. Something indescribable floats across his expression, but the main component is hurt, second only to guilt.

He exhales sharp and it's shaking--trembling from a mix of remorse and self-loathing. He's about to speak when Dumbledore does it for him.

"Young Mister Black arrived in my office a few nights ago with something that will change the entire tide of the war, but that means now I must tend to it. I leave him in your capable hands."

It's like he knows--Regulus takes Sirius' hand tentatively, like he thinks it's a trap even though now his brother looks happy--and takes a step forward that thunks hard on the floor with his cane. "Sirius," he breathes, and it's a little hoarse.

Once he's sure they're going to be able to work things out, Dumbledore takes his leave.