Well, I was reading through various fanfics on the site, and i couldn't find one that really described the incident with Snape, Lupin and the Whomping Willow. So, I thought i'd have a crack at it ;)
Disclaimer; I don't own Harry Potter or anything to do with it. JK does, that wonderful woman.
Sirius Black strode over in the direction of Severus Snape, a playful glint sparkling in his eyes. He leaned on the desk, staring at Snape, currently packing away his books.
"What do you want, Black."
Sirius stepped back in mock offense, hand across his chest. "Well, Snivellus," he gasped dramatically, "I was going to tell you a secret. Quite a good one actually, if you ask me. But if you don't want to know, then fine." He smiled.
Snape rolled his eyes. "And what secret would this be? And why do you think I would be interested?"
"Well," Sirius stepped forward again, inches away from Snape, "It's about the Whomping Willow. See, I've heard that you can make it go still. And when you do, there's a passage, a tunnel, and it leads somewhere fantastic. As for why you would be interested, well, i heard all this from Remus."
Snape's eyes snapped up to meet Sirius'. Remus Lupin disappeared every month, without fail, and Snape has often wondered why. Perhaps this tunnel held the answers? Then a thought occurred to him.
"Why should I trust you, Black?"
Sirius raised one eyebrow, and his eyes flickered to the other side of the room. "You don't have to," he said, not looking Snape in the eye, "I just though you might like to know." With one last look at Snape, he was gone.
Severus sat back down in his chair, pondering the information. He watched as Black returned to his friends, when he noticed an absence. Lupin was missing, again.
That was enough. He simply had to figure out why.
"Soooo..." James said, sprawled across his bed, his face lit by the moonlight outside, "We going to visit Moony tonight?"
His friends nodded from their respective beds. Peter reached down under his bed and pulled out their Map. Muttering the password, he scanned it. Everyone was returning to their dormitories now. He looked across the castle, following their escape route with his finger. Just as he was about to confirm their path was undisturbed, he noticed a dot out of it's usual position.
Severus Snape was heading to the Whomping Willow. Alone.
"James, Sirius, come and look at this."
The two boys lazily rolled off their beds and walked to where Peter was sat. They looked to where his finger was pointing, following Severus' movements.
"Where the hell is he going?" James asked, confusion spread on his face. They both looked to Sirius, who was sitting next to them and was for once incredibly quiet.
"What's up with you?"
Sirius' eyes slowly met James'. They were glistening with something close to fear. He spoke, his voice dry and whispered. "Oops."
Confused, Peter cocked his head to one side. "What did you do?"
"I..." Sirius seemed to find it difficult to put it into words, "I told Snape..."
"Told Snape what?!" James snatched the map from Peter's grasp, and was now on his feet. Sirius looked up to him. "I never thought he'd actually do it..." he said quickly, but James just asked him again. His reply was spoken to his feet, and was barely audible.
"I told him how to freeze the Willow. I told him how to get to Remus."
Peter stared in disbelief at his friend, but James seemingly didn't have time. He was already out the door.
"Where are you going?" shouted Sirius after him, following him down the stone steps. James stopped abruptly, and turned to face Sirius, anger blazing in his hazel eyes. He spoke quietly, as if to control his rage. His words made Sirius shiver.
"To fix your mistake. Don't dare follow me."
He watched, rooted to the spot, as James ran away. He heard footsteps behind him, and turned to see Peter staring at him, his brow furrowed in anxiety. "Sirius," he whispered, his voice shaking, "What have you done?"
James was just leaving the castle as the thrashing branches fell still, and a cloaked silhouette disappeared beneath one. He tried to quicken his pace, but he was already running at full speed. He didn't dare shout after Snape, for fear of alerting the teachers. He just had to catch him in time.
He reached the tree, whose branches were beginning to reawaken. Quickly, he tapped the knot on the trunk, and dived into the entrance of the tunnel leading to the Shrieking Shack. He crawled, doubled over, through the winding passage, the muddy walls crumbling at his touch. Near the end, he just made out a figure, and he reached for it. The figure let out a cry.
"Severus, we need to get out of here. Right now. Please, trust me." James could just make out Snape's head in the blackness. It was laughing.
"You really think I'm going to stop now, Potter?" Snape shook himself free of James' grasp, "I told you I would find out where your little friend disappears to every month. You can't protect him now."
And with that, he turned and advanced around the last turn of the tunnel. James followed, calling Severus' name, warning him they had to leave. He too, turned around the last corner, and banged into the back of Snape's frozen body. He peered over his shoulder, seeing what Snape could see. It was a fully grown, fully transformed werewolf, biting it's own tail.
Snape began to stutter as James pulled him away, back through the twisting labyrinth leading back to the castle. He pushed Snape into the cold night air and then pulled himself out of the muddy passage. He sat up, and looked over to Severus, who was shaking with shock. He reached out a hand in an attempt to comfort the boy, but Snape flinched away, drawing his wand, a maddened look in his empty eyes. They were dating to and fro, always resting on James, as the boy held his wand at James' chest in a shaking hand. James held his hands up, about to explain, but Snape cut him off.
"Murderer!" he shouted, not caring who heard, "No doubt you were in on this, Potter! You, your friends and your pet! You think I wouldn't find out? Oh, wait until the school hears about this!"
"That won't be necessary, Mr. Snape."
The calm voice of Albus Dumbledore cut off Severus' rant. He looked at each of the two boys, then walked back towards the castle, motioning for them to follow. Obediently, they did, Severus still shooting panicked side-glances at James every few seconds.