Haven't written in a looong time... but after reading OOTP, I got this idea, even though it's basically been done 92834934 times.
This takes place right after 'Snape's Worst Memory' where Harry contacts Sirius and Lupin. Basically Sirius/Remus going on (or did).
Fire
As if Sirius Black had nothing better to do (actually he really did have nothing better to do, cooped up here in Grimmauld Place), he made a big deal of searching every cold, filthy room after another for that damned elf, a process which practically dragged daggers across his skull in constant disgusting reminder of how much he hated this house. His lovely mother had died years ago, why couldn't Kreacher have kicked the bucket too?
He felt his heart lift as soon as he saw Remus Lupin bounding into the old drawing room, getting him a bit too excited, though he couldn't understand why since he always seemed to be here, since he was a member of the Order and this godforsaken place Headquarters. It was a lot better since Remus hung around the most, this fact making the house that much warmer and more comfortable.
Out of surprise, however, he hit his head hard on the mantelpiece (under which, in a spontaneous decision, he was checking up the length of the chimney, wasn't his fault that the elf liked to hide in bizarre places), and when informed that Harry had just appeared in the fireplace downstairs, Sirius's stomach gave a lurch of excitement.
Before long, they were, speaking to Harry face to face. Before long, they were back in 1975…
"I'd better go!" said Harry's head before vanishing from the fireplace. Emerald flames illuminated the kitchen with a final roar before returning to their natural colors. The crackling of the embers seemed louder than usual in the short silence that fell between Sirius and Remus.
"I can't believe this," Remus started, clearly irritated, "Snape knows why he's got to do this, he can't just back out because he was bloody embarrassed-"
"I know," Sirius interrupted, smirking, "Always was and will be a git no matter how often he washes his underpants now-"
"-Occlumency is the most important thing for Harry right n-"
"I know, don't worry, we'll get this cleared-"
"-personally by Dumbledore, I don't understand why he doesn't just-"
"I KNOW, Remus, I know," Sirius hissed. "Snape'll be teaching him Occlumency in no time after I'm finished with him-"
"We already discussed this, you're not going anywhere," Remus said wearily, "If anyone's going, I am-"
"But you're a werewolf."
"But you're an escaped mass murderer from Azkaban."
Neither of them said anything. Then Sirius laughed, looking into the other's watery-brown eyes, gleaming from the flames. Interesting how fire plays with shadows.
"Funny what we've grown up to become, huh?"
Remus rolled his eyes. "What you've become. I've always been a werewolf…" he trailed off.
"Good thing for us that you are, too, huh? What else would we have done with all those full moons, studied for O.W.L.'s and N.E.W.T.'s?"
A curious expression came over the brunet, an odd mixture of a frown and a smile playing on his lips, telling Sirius he most definitely would have taken those nights each month to do just that but could not agree more with him.
"That was a good one, using Snivellus's own hex against him... I've never seen anything so filthy, have you?" Sirius smirked, referring to Snape's underpants.
Remus smiled halfheartedly. "I don't remember that time too well-"
"'Course, you were too busy reading to notice-"
"But you've seen Peter in the bathrooms, haven't y-?"
"On accident! Merlin, don't even remind me…" Sirius frowned while the fire flared about before them, a brackish taste automatically forming on his tongue. He didn't want to think of that cowardly, worthless, whatever other words there were to describe that shit-faced traitor right now. He couldn't give more than two particles of shit about him.
"I know, I know," Remus said, laughing, although slightly bitterly. "But back then, he was just Peter. Remember that."
"Yeah… I guess he was. And James was James."
"And we were… just us."
Another silence crawled into the room.
Sirius pondered the sound of the word 'us' before Remus muttered at last, "Well, remember when we got all the suits of armor to do one great ballet in the Great Hall?"
A huge grin broke across the black-haired male's face. "How could I not? Filch was pissed-"
"Yeah, 'cause we nearly got his dear cat trampled…"
"Speaking of Mrs. Norris, it was great when we put her up in the Whomping Willow-"
"Now that wasn't funny at all," Remus grimaced, "she really could have been killed-"
"Would've given Filch an excuse to get a better looking cat at least… too bad…" Sirius said reminiscently. "Oy, remember when James bought all those mice from the Magical Menagerie and enchanted them to-"
"Sing that stupid love song about Lily he wrote, yeah? Ah yes, the prat… what a nightmare…"
"Sounded like a hundred midget banshees on fire in the Common Room, too bad Lily was terrified of rodents…"
"I liked the time when you and him turned the whole lake into a giant bubble bath," Remus said, grinning again, running a hand through his slightly greying hair. "And pink and purple, too, how'd you manage that?"
Sirius sighed in satisfaction. "Stole it from the Prefects' bathroom. You gave me the password, remember?"
"I didn't give it to you, you tricked me into telling-"
"You know what time I liked, Moony?" Sirius interjected loudly.
The brunet tore his amber eyes away from the dying fire, at which he'd been blankly gazing in nostalgia, and glanced at the male opposite him with interest.
"What?"
Sirius closed his grey eyes. "That time… when I first said it."
Another sheet of silence draped over the two, and at Remus's lack of response, he continued.
"James had Quidditch that evening and Peter detention… so I had to say it then… We spent hours there and then James came hurtling through the trees on his blasted broom telling us we'd be late for Astronomy... We were on the lake right outside the Forest, your favorite spot, remember?"
He looked at Remus expectantly, but was surprised; he'd turned back toward the fire, bitterness and tiredness in his eyes again, as often was these days.
"What?" Sirius, this time, said.
The response was barely audible, yet the cold change in his tone was clear. "'Course I remember."
"What's wrong, then?" He furrowed his brow, inching over across the cold wood floor. "Everything else was a good, fun moment… why not that?"
"It's- it's not that."
"Don't tell me you didn't like-"
"I said it's not that," Remus spat.
Sirius was close enough to put his hand on the other's wrist, and to his relief, the brunet did not pull away. Always like him to remain calm, composed in any situation-
"Then what?" he whispered. "The past isn't something to be afraid of, Moony." His pale face was right next to Remus's equally pale face.
"We can live it all over again." Their skin barely touched, and he could hear him breathing.
"Just me and you here…"
Sirius leaned in, finding and closing the fractions-of-an-inch gap between them, and a glorious fire erupted deep within his chest, a blaze pleasantly scorching every molecule of his being. His other hand urged the other male to come in closer, his fingers burning from that certain heat he hadn't felt in years, his lips opening the other's mouth. His tongue searched, gentle yet hopeful, and ever so cautiously no matter how wild the flames in him wanted him to be, and he thought he felt the smallest, tiniest response, an infinitesimal lick, the beginning of a return kiss about to fight back-
A hand shoved the black-haired male forcefully backward, and the fire died as soon as it came.
"What the bloody hell do you think you're doing."
Another silence crept in, unwelcome, as the embers sparked once more.
Sirius stared at Remus, who'd jumped up wiping his mouth, in disbelief.
"I... Moony-"
"You think this is all just a fantasy, don't you?" Remus scowled, quietly yet dangerously. It would have been better if he yelled. "This isn't Hogwarts anymore."
At loss for word, all Sirius could do was stay on the floor, painfully aware of how chill the floorboards really were, how numb his fingertips were, as Remus towered over him.
"Harry's probably at his most vulnerable right now, even under Dumbledore's protection. There's war going on…" he murmured. "And- and all you can do right now is- is try to-"
He didn't finish the sentence.
"All- all right," Sirius coughed, "I'm sorry- we were just firing off just about everything that happened back then, sorry if I was just caught up in the moment-"
"That's your problem, you're always caught up in the moment! But it's never the right moment!" snarled Remus, finally, exasperated.
"The bloody hell's that supposed to mean?"
"We're not in Hogwarts anymore, we're… we're in now. It's no longer the same time or place. You seem to be forgetting that one little fact."
Sirius slowly stood to his feet. "Then you seem to be forgetting we're the same people."
When they came eye to eye, Remus sniffed sharply. Their jaw lines and noses were barely outlined by the nearly extinct fire.
"Even so…" he said more calmly and almost reluctantly, "we're… time has changed us, Sirius. You can't deny that. Then and now are different. Everyone's changed."
"Not me," was the defiant reply.
Remus's eyes remained fixed on Sirius's, unblinkingly and unemotionally. The creases in his face became less pronounced, his mouth and shoulders more slack.
"Well," he said softly. "Maybe you should have."
He kept his gaze for one brief second more before turning away across the creaking floor, waving his hand once through the air, and muttering something about going home, and the room lost its faint orange glow. He'd distinguished the tiny flames, leaving nothing but ashes, a cold empty room, and Sirius, after watching Remus disappear from the kitchen, standing alone.