[Papyrus tries a couple of times to say something now, to say anything to keep it lighthearted, to stave off the awful memories and distract them both from all the hurt, but the words he's trying to say simply don't exist.

They can't simply breeze past this one or make a joke and move on.

For the first time in a very long time, the two of them are in a tough situation where the only way past it is straight through it.

Papyrus holds onto Sans until he's sure the smaller skeleton isn't going to rattle himself apart, then sets him back down. Not knowing what to do with himself, and with the couch an utter mess of fabric and stuffing, he sits down cross-legged on the floor.]

It all happened so fast, I wasn't sure what to do! I was very afraid of the human, of course, I knew they were doing all sorts of violence to the other monsters, but I knew Undyne would never give up if she were in my boots. And if I just tried extra hard to get through to them, maybe I could stop all the terrible things from taking place at all!

[Which still doesn't address why he never told Sans, but he's getting there.]