Remus jerked his head away at Sirius' touch, or at least as far as he was able to when he was bound to a chair. His eyes were locked on Sirius', desperately searching for some scrap of humanity that might indicate that the real Sirius was inside there somewhere. But if anyone was going to understand what it was like to be completely out of control, it was Remus.
The curse passed through Sirius' lips and in an instant Remus' body was filled with agony. He couldn't move, couldn't writhe with it, or get away. He withstood the pain for as long as he could, for as bad as this was, he knew a far worse pain, and he didn't want to give either of them the satisfaction of hearing him scream. The strain of holding back was obvious as his jaw clenched tightly shut and his face grew redder and redder. Thankfully, it only lasted a few minutes and then the curse was lifted, leaving Remus panting with the effort it had taken to hold back.