Realizing that some kind of ridiculous stand off was going down, John quickly shifted position so he could stand up straight himself. For a brief moment he debated standing on his toes to make himself taller, but for fear for Karkat noticing and calling him on it John opted to leave both heels firmly planted on the ground. For extra measure though, he put on his best "fierce" face, which probably would have been a lot fiercer if the muscles in John's face were used to the concept of scowling, and his teeth didn't stick out so much, sort of ruining the affect of those impressive flat line expressions Dave was so good at pulling off.

"Would not!" John corrected him quickly, just to make it clear he was not backing down from the challenge, even if he lacked any good retorts to something like that. He'd used up his best line already and was sort of left with hasty, and less than impressive sounding 'nu-uh!' type replies at this point. He made a note to himself to work on his comebacks for the (what he was sure would be numerous) future run-ins with this Karkat kid.