Sam didn't know what was going on, he really didn't. But Castiel had asked him to come along and it looked like something bad was seriously about to happen. Exchanging a look of concern with Dean, Sam watched as he and Ruby talked to one another.
"Wait -- human? What are you...?"
Sam didn't have time to get the rest of his question out though. Castiel started asking for them to hold Ruby back. Sam wasn't much use physically, so he leaned back against the Impala and nodded toward his brother.
Ruby was strong. If she tried to fight Dean, he'd contribute with his abilities. Hopefully it wouldn't have to come to that. Sam was already tired.