Wow. Excellent portrait of the two men, with palpable, dense atmosphere, and the characterisation through their silence is fantastic.
Just a few examples of lines that gripped me particularly:
Moody doesn't waste words, but his body language is eloquent. As the weeks pass, Remus learns to read the silences between them. On bad days Moody stares too long into shadows, fingers gripping his tankard, and shoulders hunched. On good days his hands lie open on the table and he meets the gaze of the other patrons, his magical eye skittering from face to face. Moody never invites anyone else to join them.
It's hard to identify the line between constant vigilance and a different kind of interest.
Everything seems to happen in slow motion. It's nothing like the sex he had before the war, stilted instead of rushed, not laughing but silent except for the squeaking bedsprings.
Even within the walls of Moody's house they never kiss; Remus knows how Moody's mouth feels on his cock, but not on his lips. He doesn't ask why.
Remus can't imagine anyone better able to survive. There's a lesson in that, if only he can find it. -- Wow.
The father/son moment at the beginning is great, too -- sets the theme, the mood, the leitmotiv, as it were ...