Having to just sit there and do nothing was without question the hardest thing Remus had ever done. As soon as he heard the incantation Severus used, he knew what was going on. Legilimency. It was such an invasive form of magic. Ingenious, really, and Remus had no idea Severus had reached such an advanced level of magic. It surprised him, and it scared the hell out of him too.
More than anything, he wished he could trade places with Sirius and be the one who was going through so much hell. He would've given anything to be able to take away the pain and suffering he'd endured. As it was, all he could do was sit there while Severus hopefully was able to reach him and solve the problem.
When Sirius started screaming, Remus' eyes widened and it took every ounce of willpower he had not to move closer and shout at Severus to stop. He had to get rid of whatever it was in him that was causing all this suffering and strife. Remus clutched at the fabric of his trousers so tightly, his knuckles had turned white. The name Sirius shouted filled Remus with dread and fear. Voldemort? He thought. He looked on with worried eyes, both afraid for Sirius and Severus at this point.