I believe "You will eat lunch / at school / with friends" most clearly develops an argumentative tone because of the 'will'. Sounds like someone's trying to force the person to do that and the person may try to argue back because the will makes it sound like they have no choice in the matter.
A flat character is a character who is 2 dimensional and does not change throughout the story/book. Hope this helped! Have a great day! :)
