Sirius didn't think he could take the back-and-forth much longer, the struggle between Bellatrix's overwhelmingly strong presence in his mind and his own self, the vague awakenings of his conscious that had been quiet for far too long. His focus kept coming in-and-out, expression changing from certain one moment to confused the next.
Then Bellatrix's voice was urging him to drive the knife through Remus' foot, to make him pay and it was too bloody strong to ignore, and maybe if he just gave in it'd stop. He picked up the knife and in one movement drove it straight through Remus' foot, the tip sticking in the ground beneath them. Bloody welled up and poured from the wound, and something in Sirius' snapped at the sight of it, covering his hands.
"Stop!" he shouted, yanking his hands away from the knife, from the sight of it, realisation crashing into him hard. For the first time since the curse had been put on him he looked like himself, eyes wide and horrified, the full impact of what he'd just done hitting him hard enough to make his stomach twist and leap into his throat.