Disclaimer: It all belongs to J.K. Rowling
Sometimes, when he is sitting in the damp dark of the cave at night, Sirius smokes a single cigarette. Never more than one, because they are hard to get, and he can't count on a steady supply. He doesnt't light a fire at night, since the light could shine from the mouth of his hideout and give him away, so he sits in the dark and watches the orange cinders glow on the end as he takes slow pulls of the rich smoke.
Remus brings him the cigarettes whenever he comes to visit. These are rare occasions, and they are becoming more and more rare. Sirius knows Remus is doing dangerous things for the Order, but Remus won't tell him about what those things are. He doesn't want to burden Sirius with worries about his safety, but the not knowing makes Sirius worry more. When Remus does come to visit they smoke together, sitting side by side at the mouth of the cave and staring off at the orange glow of the sunset. Sometimes Remus blows smoke rings; Sirius was never patient enough to learn the skill.
When they kiss after smoking, Remus tastes like tobacco and chocolate. He tastes like the past.
When they were at Hogwarts together, Remus and Sirius used to smoke in bed, naked and tangled together after sex. Always in Remus' bed, because Sirius was the one brave or foolish enough to sneak silently across the room, careful not to wake their friends, and curl in beside the other boy. Once on a whim he'd brought a packet of cigarettes; he'd seen an old black and white Muggle movie and thought the curling smoke looked debauched and glamorous. Remus had been uncertain at first, and they'd both coughed violently, stifling the sound in pillows. But the habit had stayed, grown on them as a tradition.
Eventually Remus had started buying his own cigarettes at a hole-in-the-wall shop in Hogsmeade. They were stubby and smelled richly of cloves and something spicy that Sirius couldn't identify. But he grew to love the scent, and the taste of it on Remus' lips.
When they kissed, enclosed in the drapes of the bed, Remus tasted like spices and chocolate and Sirius. He tasted like the present and the future.
At Hogwarts, Sirius never smoked alone; it was something he and Remus shared, something intimate and secret. But in the dank, claustrophobic cave, he smokes alone all the time. He drags out every cigarette, watching the brightness of the burning end wax and wane with his breath. When he's smoked it down far enough to feel the heat on the tips of his fingers, he watches the last spark fade and flutter out, then sits alone in the dark.
The remnants of smoke on his empty lips taste like the future, but they're bitter and stale.