Phil is a collected, manipulative character, shown through “Please keep up, I’m making this up as I go along.” in Act 1. This means that the audience can decipher the author has given him dimension of authority within the play. Although he is silent or preoccupied for the vast majority of the play, Phil reveals himself as an evil mastermind, adding further to the impression of mild teenage sociopathy.
“If you don’t help us, we’ll kill you. Are you going to help us?” in Act 2 confirms this, through the idea that Phil attempts to appeal to the herd mentality in Brian, through the threat of physical violence as opposed to regular persuasion. This makes him seem dangerous and unstable.
Phil, although potentially quiet and reserved, turns out to be power–hungry and violent, and asserts himself as a leader through proclaiming “I’m in charge. Everyone is happier.”- He takes the power away from the rest of the group to assert whether or not they’re actually happy, but rather decides for them, which diminishes their autonomy as individual human beings, and asserts him at the centre of power and attention- something he’s bound to enjoy as a teenage, power-hungry sociopath.