Just a fun little one shot I could see occurring. Set during the Marauder's fourth year at Hogwarts.
Lupin was interrupted from his reading by the sound of clattering footsteps and the clang of the fat lady's portrait closing. He looked up to see James, Sirius, and Peter enter the common room. It was midnight and until then the room had been empty except for Lupin. He noticed that James and Sirius looked very pleased with themselves, whilst Peter was flushed and looked a little jittery.
"Alright, Moony?" James said, sporting the grin that told Lupin his friend had been up to no good. James flopped down on the longest sofa, Sirius followed, resting his feet on a spindly table, and Peter sat meekly on the sofas arm.
"Where've you been?" Lupin asked suspiciously.
"Show him, Padfoot," James said.
Sirius grinned and from beneath his cloak he produced a full bottle of fire whiskey. "Nicked it from the staff room. Took us about two minutes, which, and correct me if I'm wrong boys, is a new record?" He said.
"Our fastest yet," Peter agreed breathlessly.
"Who fancies a swig then?" Sirius said. He tapped the bottle with his wand and the cork removed itself with a pop.
"After you," James replied cheerfully.
Lupin sighed. A lecture was itching on the tip of his tongue, but he knew it would be pointless to voice it. Sirius hissed a little as the whiskey burned his throat. He chucked the bottle to James, who caught it deftly with a Seekers skill. He took three sips, winking at Sirius as if to prove a point, and offered the bottle to Lupin. James could see the hesitation in his friends pale eyes.
"Aw come on, Moony! Have a little fun," He said.
"Take a walk on the wild side!" Sirius added.
Lupin sighed. He had found that over the years the rebellious nature of his two friends had rubbed off on him slightly, and he found himself reaching for the bottle. He took a large gulp and tried to ignore his watering eyes. He grinned, wiping his mouth on the back of his hand. "I can out drink you two any day!" He jeered.
Peter leaned in eagerly for his turn, but he was now out of the game. War was about to rage between his three friends, and he knew how messy it was likely to become.