Here is the Third Year Incident for ramen_addict, which I have mentioned several times in my own personal canon. While it is not as long as you probably would like after all that build-up, but I feel like I essentially told the story.
Backing Up Your Threats
"And then I said to her…"
Remus looked up when Sirius' story stopped abruptly, even though it had clearly been headed somewhere questionable. He glanced around for a moment, before spotting him harassing a group of Slytherin First Years.
Including, or more accurately because of, Regulus.
Remus ground his teeth as James joined in as well, he and Sirius taking turns ridiculing and shoving.
"Stop it, both of you!" Remus snapped finally. He stormed over and yanked both James and Sirius away by the arms, dragging them down the hall. Peter was so shocked, he stood rooted to the spot for a few moments before running to catch up.
"Oi, what's up with you?" Sirius demanded, ripping his arm out of Remus' grasp.
"You," Remus snarled at him, and turned to James, "and you! It's ridiculous the way you go after Regulus like that!"
"He's a slimy little Slytherin," James said dismissively. "They're made to wipe our feet on."
"He's Sirius' brother!" Remus pointed out.
"He's a horrible little shit," Sirius bared his teeth. "Always following me about, snitching to mum in his letters…"
"He isn't following you about!" Remus shouted. "You're seeking him out to torture! I'd write my mother too if some arse of a bully were shoving me around all the time!"
"You don't know my mum," Sirius' expression closed down completely. "You don't know what I'm talking about."
"Fine, you're right," Remus snapped. "I don't know anything about it, but what I do know is that if you don't leave Regulus alone I'll stop speaking to you."
There was a moment of silence.
"No, you won't," James said with authority.
"That goes for you too, Potter," Remus narrowed his eyes. "Since you two can only manage the social skills of five-year-olds, I'll have to resort to sinking to your level."
"Stop it," Sirius said curtly.
"If I catch you doing it once more," Remus stared him dead in the eyes, "I won't speak to you for a week."
Remus continued briskly down the hall as if nothing had happened, leaving Sirius, James, and Peter staring at each other in mute surprise.
"He doesn't really mean it," James announced at last, reasserting his leadership.
Two days later, when Remus caught Sirius dangling one of Regulus' books high over the First Year's head, they found out for sure.
* * * * * *
"Sirius, you've got to do something," James ordered.
"Me?" Sirius glared at James over his half-done Potions essay that was due the day before that he'd forgotten about because it was Remus' turn to do the Potions essays but clearly he wasn't doing their homework this week. "Why me, he's just as angry at you!"
"Sirius, we're all going to fail our History of Magic exam if you don't make him listen to reason," James pressed. "Remus is the only one who can stay awake during those awful lectures!"
"Looks like we're fucked then," Sirius reported, going back to his essay.
"I've tried everything," James continued. "I've tried asking and apologizing and ordering…I even had Peter go and beg on knees, but it's no good!"
Sirius merely grunted, scratching out useless lines of his essay with broad quill-strokes.
"Why are you being so calm about this?!" James finally shouted, snatching Sirius' essay off his desk.
"Listen, didn't you believe him when he said he'd stop speaking to us?" Sirius asked.
"No," James admitted. "Not really. I thought he'd cave after a bit and do what we wanted, like usual."
"But he didn't," Sirius pointed his quill at James. "He made a threat and now he's sticking to it. There's a sort of honor in that."
"Blacks," James snorted, throwing down the essay parchment in disgust. "You're all twisted. He can't last the whole week, he won't let us fail this exam, I'm sure of it."
"Five galleons says he does," Sirius said.
"Ten," James countered.
"Done," Sirius nodded.
The following week, when Binns handed back one A and three big, red F's to the Marauders and Remus turned around in his seat and asked, voice rusty from disuse, "How did you lads do?", Sirius realized he was completely smitten.
And ten galleons richer.