Sirius itches like the delicious sensation when you peel the top layer of skin off after a sunburn. Remus can't stop scratching, picking, peeling at him. Remus can't let him just be, no more than a sunburned teenager can stop themselves from the lurid appeal of peeling their skin off like parchment, rolling it into tight balls and peering through sheets of it. Sirius never sits still, never seems to settle and heal, is always endlessly fascinating and eternally painful.
Sirius flares like the Muggle fireworks he pretends to enjoy just to piss off his parents and the thousands of other purebloods he's forced to interact with at Hogwarts. He's shoddily made in some ways, and strangely beautiful in others. Wizarding fireworks are more colorful, last longer, and are safer by far. Muggle fireworks are dangerous and half-cocked and bright and explosive, and they're too loud and too fast and over too soon. Sirius isn't safe in any sense of the word, and he's more beautiful because of it.
Sirius burns down the throat like firewhiskey, like a bad shot of something green and viscous and illegal. He's like those gallons of blue drinks Muggle girls love, the ones that are swimming in liquor but taste like candy, so the drinker downs a dozen without realizing it. Half a dozen drinks in, Remus realized how wasted he had become, how addicted to Sirius he was now — but it was too late by then.
Sirius smells like cinnamon and wolfsbane, and he always makes Remus sneeze. His skin never smells like soap or cigarettes or sweat, despite Remus having observed Sirius showering, smoking, and sweating all at the same time, more often than he is comfortable disclosing. Sirius's scent is subtle and soft but it's there and unmistakable and imprinted on everything he owns. Remus could walk blindfolded into a garbage pit an pick out what Sirius had contributed without missing a beat, because he's smelled that scent on his own skin a million times.
Sirius snores like an old man, lying flat on his back, usually in Remus's bed because his own is littered with socks and rubbish and cigarette butts and paperback novels he nicks from Muggle bookshops when he gets antsy, which is every few weeks. Sirius snores and whuffles into Remus's neck like a dog and sucks all the warmth out of the room with his fucking frigid feet and steals all the blankets and drools on Remus's pillow.
Sirius fucks like a teenager, all hurry and passion and fear of getting caught and excitement at the idea of getting caught, teeth buried in Remus's shoulder and hands scrabbling at Remus's back and moans and keening cries and shaking, sweating, shivering orgasms that seem to go on forever and be over in an instant. And then ten minutes later, he's laughing and shimmying around the room to whatever Muggle band he's in love with that week and pulling Remus up to dance with him, full of afterglow.
Sirius bites like a werewolf.
Sirius hides like a werewolf.
Sirius changes like a werewolf.