Well, this isn't looking promising for some big energetic kerfuffle in which Sans-MoI is distracted by other people, but it does have him? them? turning and walking somewhere else. Obviously, Papyrus isn't willing to let that just happen like that. So he shifts the bones to one hand and raises the other like he's in class, following after his possessed brother.

"Uh, excuse me, scary person walking around in my brother? We've let you speak, and offer a deal... so, can you go away now? It can't be good for him for you to be in there. I know I'd be harmed by somebody walking around, in me, instead of me."

Mostly because, from the times anything like that happened before, they controlled him into getting in fights where he was at least a little injured. But the morale blow counts too.