It almost scared him to see Sirius so resolved to the idea of Snape helping him. Ordinarily he likely would have protested to the point of cursing and yelling, or just outright refusing. But he'd barely even given a response. Remus knew this was their last chance before he'd have to get someone like Dumbledore involved.
He wished he could be the one to give Sirius an answer, but he didn't have any. Those would have to come from Snape. So all he could do was smile reassuringly at Sirius and sit next to him on the bed, then turn back to the Slytherin for answers.