In which Sirius is (repeatedly) surprised.
The chapter title references Terry Pratchett's The Truth, not that movie you might be thinking of.
They had nearly a full day of peace. If one allows that even horrible smells, mouldering piles of junk, rotten wallpaper, cursed heirlooms, and small biting magical vermin can be a part of peace, when taken in good company and without injury, even without anything stronger than pumpkin juice to soften the botheration and itching.
Eventually, though, Snape came back from the castle to spoil what peace there was to be spoiled. Under his arm was a bottle full of clear, faintly yellow-tinted liquid, petals and supple green needles floating in it. Sirius, full of unsettling dreams and yesterday's fury and several lungfuls of filthy black dust, could not possibly have cared two fewer pins what it was, or why.
Snape didn't seem to be deathly ill this time, so although he did look like grim, lowering death having a bad day, Sirius felt no compunctions about getting in his face and growling, "Remus may be the most forgiving person in the entire world, but let me tell you, Snape, you are going to regret the day—"
"Yes, yes," Snape cut him off, with a lazy drawl and an airy gesture. His nostrils flared testingly, and then there was a flicker of relief over his face that made Sirius shiver clear to his boots with rage. The hand that had punched the bedboard in Sirius's sleep last night clenched, drawn with magnetic longing to its favorite beaky target. Without pause, though, Snape finished, "I must wear my rue with a difference."
This stopped him short, a soft slap full in the face with a silky bag of petals. He knew this, he knew it, the grass-vanilla-dust-and-leather smell of old books, and Remus's low voice, lulling them all to sleep… After a moment of processing, he was able to tell Snape, in a much calmer tone, "Go jump in a lake."
Snape smirked, and presented him with the bottle. He angled his angular chin slightly to suggest posing, and declaimed, with a wicked eyebrow, "Here's rosemary, that's for remembrance; I pray you, mutt, remember that I can supply your deficiencies any day of the week. And pansies, actually, and fennel, and columbine—a soppy name, bindweed suits it better. No extract of almond, though. Since you haven't Lupin's sweet tooth I imagine the honey medium will be cloying tedium enough, on the dry side though it is."
Sirius had the feeling of being stopped short and doused with something shocking. Again. He stared at the bottle, which his hands had somehow closed around, and managed, "You made me mead?"
"Metheglin," Snape corrected him like the pedant he was, as though it mattered. "It's called metheglin, when it's herbal. If we put the apples and bananas in, it would be malomel. And don't flatter yourself; the base was already made. It's nothing to drop a few herbs and powders in and haste the infusion. I suppose we won't get you off the firewhiskey cold," he added sneeringly, but with a glint in his eye that looked secretly pleased with himself.
Sirius stared from him to the bottle some more, and finally asked, "Why?"
Scoffing, the complete prat answered, "Oh, please. 'Goodness gracious, has Gryffiths left the Harpies? Mercy me, and probably lawks.' You forget I've known your family since I was eleven. I know what it looks like when a Black tries to drop to his knees and beg."
"Fucking gorgeous is what it looks like," Sirius fired back automatically, instantly wanting to avada himself for tossing his head like he and his hair are still young enough for it not to look completely, pathetically stupid.
Only milliseconds less instantly, he's blown away for the third fucking time in five minutes, godricdammit, by the arrested look in front of him, the way the slight widening of black eyes makes them suddenly look blown-pupiled instead of sink-holes, the way the softening of the lines that shoot down the forehead and imprison the severe mouth soften it, too, make it a mouth instead of a harsh, impatient line. There's a feather-touch by his eyes again, Merlin, drifting slowly to his lips, again. A dark voice murmurs, as though it doesn't quite hear itself. "Thunderheads. Night water. Singed mercury."
Sirius's mouth opens, to—what? To say something? Or to… but it doesn't matter what, because Snape is flinching, letting his arm drop, not clutching but flinching around it. The lines on his face are back, with whole exhausted families of company.
"This is often," Sirius says, his eyes on those two harsh runnels above Snape's eyes. They had almost looked, a moment ago, as though they might be capable of thinking about considering a leave of absence. He finds that his hand has wrapped without his direction around an arm feverish with dark magic, that his smallest fingertip has insinuated itself under an overly-long, overly-starched white cuff to rest lightly over a suddenly jackhammering pulse.
"Yes," Snape says blankly. Then he does it again, this time leaving Sirius not stilled or electrified but chilled. Producing his mask and cloak from one of his eighty billion pockets-of-holding and putting them on, he says in a flat voice, "There's something he wants retrieved. It's not now where he left it. Tell the Headmaster that; I haven't seen him since this morning. There are some works in progress in the top left drawer of my desk you might find useful, if," the pale silver scrollwork covers his indifferent face with an almost audible click, "the little bastards have left me a desk."
And then he's gone. Sirius stands very still, clutching the bottle. Breathes. Considers swearing. Considers drinking. Breathes in, deeply, and shouts, "Winky!"