Where would we be without you. [Nathaniel says dryly, making it very hard to tell if he's being sarcastic or not. When most of what you say sounds like you're trying very hard to seem like you mean it sincerely, such things do become a struggle.

At the further explanation, though, he does straighten up in his seat and a frown settles on his face.]

You know, that does sound like something you should have brought up sooner, but I can see how it might have slipped your mind.

[... And that is sarcasm.]