"Ah, Filch, there you are." Snape said, in a disturbingly friendly voice, appearing from a cross-hallway.
"Professor Snape! Would you happen to know how Mister Potter has managed to coat the entire floor and most of the walls in stinking mud?"*
"I would, in fact." Snape said, smirking.
"Well? How did he manage such a feat? Lad claims it was his detention..." Filch said skeptically, giving Snape the hairy eyeball.
"And so it was," Snape said, moving through the muck without hesitation. "As a rather fortunate coincidence," Harry swore he saw Snape's eyes twinkling, "I happen to have some other detentions tomorrow morning."
Filch looked marginally happier, as if this was a normal thing, and he could see something coming that Harry Potter couldn't. "Izzat a fact, sir?"
"Indeed," Snape said, "How would you like to take tonight and all of Saturday off?"
"That would be most agreeable, sir." Filch said, and tried to look happy. It was a foreign look on his face, in so far as Harry had ever seen it. But then again, it looked like he was always on duty, and cleaning up after an entire castle - seven floors, plus the dungeons, plus the towers on top? That was not a job that was exactly easy.
"Tomorrow, my Slytherins will learn how to clean without magic." Snape purred, his eyes sparkling.
"I'll have the mops ready," Filch said.
"Bring a toothbrush as well," Snape responded.
"Is someone cleaning the Owlry then?" Filch asked, excited. As far as Harry knew, no one had ever cleaned the Owlry. Though it probably wasn't entirely Filch's fault, as owl droppings were everywhere, giving the delightful crunching feel of small bones breaking under your feet, whenever you went through there.
"Indeed." Snape said, "Potter, return to your dormitory at once."
"No sooner said than done." Harry Potter said, hurrying off before either of them could come up with something else for him to do... after curfew. Besides, he wanted to be up early to see the looks on the Slytherins' faces! This was going to be good.
*Methane reeks, doncha know.
[a/n: Snape's detentions run long.
The visitor that's coming is the reason why the Slytherins have this particular detention (that was scripted nearly at the same time as I wrote in the visitor coming).
Harry's detention? That got scripted about a chapter ahead of it actually happening. Ditto Filch showing up.
Snape was, obviously, heading over to tell Filch that cleaning up this mess wasn't part of his job.]