River watches anxiously, because there's very little else that she can possibly do in this situation. Her weight would have toppled the floor. If Jamie died again, the Doctor would be quite cross with her, so when he makes that jump she could feel her heart leap in her chest. The moment her fails to land properly, she darts forward and grabs his shoulders. This way he can pull himself up with his hands and she can pull him forward by her grip on his shoulders.

"Come on, Jamie. Show me what scotsmen are made of."