You probably think you know what you're supposed to do for this one; energetic, kind of high pitched, slightly nasal voice with a caracteristic accent. Well, you're right... sort of. He will have very few lines, and in most of them he will not be as energetic as you're used to. Instead he will be overall quiet, even nervous at some point. So basically, try to make a tiny little bit quieter Papyrus and it should be okay.