Sense And Lunacy
1) Touch
While they were deep in the woods, having outraced the stag and the poor rat, they tumbled into a bed of moss and showed each other the things they could not say as humans.
Even months before that night, Sirius had been happiest under the full moon, when Remus would think nothing of Padfoot nuzzling his fur or playfully humping his back. But that night when nuzzling turned to licking, Sirius discovered that his shy, ailing friend was neither. That night he learned how it felt to come alive, shuddering monstrously, with his canine cock between the wolf's eager jaws.
2) Sound
Padfoot can hear the wolf rising in Remus before anyone can see it, snarling howling roaring within, calling itself into being as the moon breaks the horizon like an egg cracking.
And he can hear Remus fight, clenching wailing crying against it, the struggle internal at first, then in the snap of his jaw and the crunch of resisting bone.
With a snuffle Padfoot moves to Moony, crooning quietly, sounds that seem like whimpers to Prongs and Wormtail but which the wolf can understand. The growl in his throat is familiar as a lullabye, and he pants softly in welcome.
3) Smell
The first time they both transformed, they spent so much time sniffing each other that Peter teased them uncomfortably afterwards. So they don't do that in front of the others anymore, but he'll still catch Moony racing in his tailwind, chasing his scent.
Once he secretly marked the collar of Remus' robe, spraying a jet of Sirius-musk inside the seam that held the clasp. Remus stalked him, sweating, all that afternoon. Finally he dragged Sirius into the Room of Requirement where he fucked him over a warm, undulating body pillow that appeared there for just that purpose, smelling of fur.
4) Taste
Sirius understands what it is to crave blood.
His first was his own, filling his mouth along with bitter deadly venom sucked from his wound. Then it was Prongs', licked gently and lovingly from scratch after scratch after the flight from the hunter.
Sirius has discovered Remus' blood on his lips following bruising, claiming kisses, salty like tears of regret. But he has never savored the blood of the werewolf.
And Moony has never sipped his blood, torn through the skin to the meat beneath, though Padfoot can feel the urge in him every time... the hunger in the teeth.
5) Sight
There are always bruises afterward, dark mottled shapes on the ashen skin, most pronounced where the body has had to compact itself: hips and elbows, thighs and wrists. Where Remus has been stretched, mouth and neck and calves, his flesh has grown translucent.
Only the genitals remain unchanged, though they lie flaccid and pale as the dawn. Sirius strokes away Moony's pain, then brings the cock slowly to life under hands. He looks up, watching Remus' eyes grow fierce with pleasure, and he vows that if he cannot stop Remus from being hurt again, at least he will never flinch.