In The Giver, how does the setting affect the main character's attitude and actions?

By main character, you must be referring to Jonas, and one could argue that he acts both obediently and rebelliously as a result of the setting.

He acts obediently, as all members of the society do, throughout most of the book. He follows the rules and routine of the community and does not, at least outwardly, question them. In this way, he is responding directly to the setting in which he lives. He lives in an orderly community that dictates everything he does. He does as he is told and nothing more. He knows no other setting.

As he begins to learn about the way society used to be, however, he does begin to question why the community has chosen to be the way it is now. In moving to Sameness, he recognizes that individuals gave up their freedom and ability to choose for themselves. This causes him to want to have more freedom and to want to have choices. This is when he begins to rebel against this dystopian setting he has been living in all of his life and ultimately escape it.