A/N: This is what I think likely happened when Sirius tried to get Snape killed by telling him out to get near Lupin under a full moon.
I thought this was one piece of history that needed some filling in. I hope I'm accurately representing the characters and their (possible) relationships at this time. Please read and review.
A feeling of shame and embarassment was settling in Lily Evans' stomach. Why did I snap like that? I shouldn't have stooped to their level....
As she reached the door to her usual transfiguration classroom she braced herself: she couldn't let it show that she was angry, whether or not they knew she was angry at herself, rather than at them.
She knocked and entered. But before the door could close behind her, Sirius Black entered the room, looking slightly out of breath.
"Evans, Black," said Professor McGonagall crisply from her teacher's desk, "take your seats, please."
Lily turned to the rows of desks and chairs among which two people were already seated. Snape was sitting in his usual seat near the lefthand side of the room, on the edge of the aisle. Sirius headed for his normal seat across the aisle from Snape but a few rows behind; Lily took hers directly ahead of Sirius but two rows in front of Snape.
Glancing to her right she saw the back of Bellatrix Black sitting in a seat near the righthand side of the classroom, looking out the window. As if sensing Lily's gaze, the seventh-year Slytherin glanced around to give Lily a scathing and disgusted look and returned to staring out the window.
Taking a deep, silent breath, Lily again suffused her anger (this time towards Bellatrix), and looked to McGonagall.
"Now," the professor began, "you all know what you are here for, so I see no reason to delay your detention punishments." She stood up and headed towards the chalkboard that lined the front wall of the room.
Yes, I know why were here. Sirius and Snape are stupid gits, I'm an idiot, and Bellatrix -- well, Bellatrix is just a bitch.
McGonagall tapped her wand to the board where the sentence I must not duel in the corridor suddenly appeared. "Now, I want you all to copy this sentence -"
"And how many times will we be writing it, professor?" asked Sirius in a tone of mock-cheeriness, tilting his chair backwards.
"You will continue writing until I tell you to stop," replied McGonagall icily.
Well, that's not so bad. But after she had several feet of parchment covered it struck Lily that McGonagall had chosen quite a fitting punishment. She would have much rather have been doing something like cleaning the Trophy room by hand, or something a little less tedious than writing lines. Glancing around at the others, Lily observed that the others looked just as bored as she was. Sirius was slouched in his chair, writing very slowly, apparently unaware that his left leg was streched out in the aisle, his foot jiggling as if it had been jinxed to do so; Snape was writing studiously but his large nose was quite far from the paper -- a definite indication of boredom for Snivelly; And Bellatrix had her quill poised on the parchment, but appeared to be in a daze and wasn't writing at all. Lily's hand ached and despite the fact that it was still light outside, she felt as if she had been writing for a whole night. She blinked hard and continued writing. A fine choice of punishment indeed, McGonagall.
As nighttime began to fall and the candles flickered into light there was a knock on the door. Professor Flickwick, their squeaky Charms teacher and McGonagall stepped just outside the classroom, into the hallway to have a whispered discussion. Lily took the opportunity to rub her tired eyes and stretch her arms. It had certainly been a long day.
"Psst." Uhg, what now? Lily slowly turned in her seat. "Psst, Snivelly." Who was he kidding? Why can't Sirius just leave Snape alone? She turned to see Sirius leaning forward to get Snape's attention, an expression of nothing but vicious loathing on his face. Snape was still staring at his parchment but was no longer writing, his head slightly turned towards Sirius, his face full of hate.