At the beginning of the book Jack and the choir arrive in an almost militaristic fashion with Jack shown as the leader just from the way he directed them you could tell that Jack ruled with an iron fist, which could have been based on the way Golding described him in the book on pg.16.
"Inside the floating cloak he was tall, thin, and bony: and his hair was red beneath the black cap. His face was crumpled and freckled, and ugly without silliness. Out of this face stared two light blue eyes, frustrated now, and turning or ready to turn to anger." By describing Jack as tall, thin and bony he is basically implying that all the other members are shorter them him and that he has a slightly sinister look to his features and which makes the choir view him as their leader because he has a more imposing stature. Also by describing his eyes the way he did Golding gives Jack a seemingly volatile nature that seems slightly imbalanced as if he doesn't bother to control his emotions which makes the reader think that there is definitely going to be some conflicts between him and Ralph throughout the novel because of their different views and personalities.
Jack is portrayed as Ralph's complete opposite even though they are both leaders of their respective groups. While Ralph represents the more civilized part of humanity Jack represents the savage and power hungry part of humanity. From the way he is in the book I view Jack as the type of person that always has to be in control of everything no matter what the situation and dislikes being disobeyed he also doesn't like having to share power or be second best to someone else.
I think that as the novel progresses and as Ralph starts to lose more and more control of the tribe Jack will become more and more bold and try to take over or start his own tribe. As Ralph starts to lose control of the tribe and power of the conch grows less significant I think that Jack will take advantage of that and gradually gain leadership. The only reason Jack treats Ralph with grudging respect is because Ralph is chief but he acts as if all the other members are beneath him and not worth his time so he treats them more as subjects or servants then actual members of the tribe.