Role of position coaches

What is the role of position coaches? Where is the line between what a position coach is responsible for and what the offensive/defensive coordinators are responsible for? Head coach? If for example, the linebackers were consistently not showing up and played poorly, who shoulders more blame? Ultimately it's on the head coach to make sure the right personnel is in place, but for the sake of this discussion, I'm interested in the responsibilities of the week to week preparation once a season has started.

It probably varies greatly from team to team.