At first, neither of them expect it to go beyond friendship, not again. They had the chance Before, and everything was wonderful, it was marvellous. (Until it wasn't – but what more can you expect in the middle of a war?) They don't need that now. All they need is each other's company, and they have that.
It happens anyway, sideways smiles leading to stolen kisses to backs pressed against walls and roving hands. They are changed men, re-forged from the fires of hell and yet this stays the same, this intimacy and arousal and craving for each other. And this is what gets them through these long nights when it's only the two of them in the big old house. Remus hates thinking of what they have as a distraction, yet knows that in part, at least, it is. When they're together, folded in on each other, then everything else fades away and they can breathe. It doesn't matter that he's a werewolf and he's an escaped prisoner, all of that simply disappears, years melting away as they share a bed.
Sirius always sleeps better afterwards – nightmare-less and sober. Remus mostly doesn't sleep, instead holds him close and strokes his hair, no matter how tangled and greasy, because this is Sirius, his Sirius, no matter how damaged and broken inside. (And those tattoos are new, but he's learned to deal with them.) It's enough to have him here, so peaceful. It was rare for him to be peaceful Before, always dealing with some new scheme or plan or attempt to take down Voldemort, a mind rarely at rest. But twelve years in prison have lent him a new appreciation for ordinary life, and so now he can rest in the moments – at least – when the bitterness can be forgotten. Mostly, he is bored and impatient, but the rest of the time they are fabulous, shining brighter than the star for which he is named.
(And Remus, for one, is so grateful that they've taken the next step again.)