If someone were to ask you when it was that he started smoking, you wouldn't have been able to tell them. You doubt even he could. It's something that's always been there, as long as you can remember, the smell of smoke and the ash-stained fingertips are as much a part of Sirius as your 'furry little problem' is of you. You can no more imagine Sirius without the fog of smoke than you can James without his obsession for Lily. You think maybe it's something that's grown with him, maybe he was born with a fag in his hand, maybe the ash is secretion from his skin and maybe tar runs in his veins instead of blood, perhaps his bones are built from stubbed out ends of cigarettes and his skin is smoke made tangible, moulded together and stretched thin, wrapped around his bones like bandages round a mummy, and if you were to slit his skin grey vapour would float out and he'd deflate like a balloon and collapse into a pile of ash.
A/N So this is from my oneshot You Go Out Like A Riptide (shameless self-advertising there :p) but I'm putting it in here cos I quite like it