I remember being thrilled when I saw this first come up because Minerva/Alastor is my top Minerva het pairing, and I am so glad that I finally -- finally!! -- read this. This is all I love about Minerva, Alastor, and Minerva/Alastor. And then some. Two stubborn, hurt, stalwart soldiers who bear the scars of the wars they've fought and who will never stop carrying on. Both strong and fierce, but each in their own way. You portray this so brilliantly that I can only echo for both what TRS says: They're more IC, more themselves than in canon. They're two adults with a history -- and we can see how Harry wouldn't be able to begin to understand. (See your comment on Harry not seeing Minerva's flinching).
You deliver a feast of voices. Alastor's narrative voice is perfect, and the dialogue is to die for. Minerva? At her best. Just four out of many examples that make my heart all aflutter because this is how I love her written -- she's alluring, reserved, sexy, and she scares me a bit. In a good way.
Minerva perched primly on the edge of a plain wooden chair, leaving enough space between its straight back and her own to have made any Victorian grandmother proud. The sight brought out the worst in Moody.
There was none of her trademark shouting or stalking off, none of the wayward magic that used to snap sparks out of the very air around her when she was angry.
She raised an eyebrow and let the silence ride until she saw his eyelid flicker. Damn her. After all these years, she could still stare him down, and he even had a magic eye.
"Can I ask you a personal question?" he said. "Personal?" Minerva cast an eye over their entwined nakedness. "Well, I suppose now would be the time."
Other bits I adore are the tenderness you portray, while at the same time they try to protect both their own and each other's feelings, as well as the believable explanation you offer for Minerva's failure to recognise the impostor during GoF.