When William Golding said, in Lord of the Flies: "He capered towards Bill and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness," (64) he is in the sense describing what happens to the boys. Knowing what happens, how does this description explain the change over the boys' behavior?
In Lord of the Flies, the boys on the island are quick to appoint a leader - Ralph - and to use the conch as representative of order. It is an almost automatic response to their situation as there are no "grown ups" to take control or tell them how to behave. Piggy and Ralph recognize the need for a signal fire and the importance of making suitable shelter.
On the other hand, Jack contemplates the huge adventure and, whilst he is head chorister and feels that he should hold a position of authority, he cannot wait to shrug off any hold that his "old" life may still have on him. He camouflages himself so that the pigs that he is hunting will neither smell him nor see him and his "dazzle paint" as he calls the clay he smears on himself will allow him to blend into his environment “like moths on a tree trunk.”
When Jack sees his reflection he is delighted and views himself as "an awesome stranger" basically giving him permission to behave any way he likes, free from civilization. Being "liberated from shame and self consciousness" is a release for him from the demands placed on him by society and "the mask" allows him to forget any sense of responsibility or even think about consequences. The mask gives Jack a power he lacked before and the boys with him can see the change in him.
This description then starts with Jack's transformation which will affect the others as "the mask compelled them."