From the Moving Through a Square
entry of the Pathfinder RPG Reference Document
and the Movement, Position, and Distance
section in the Hypertext D20 SRD
(for 3.5 D&D):
Moving through a Square
You can move through a square occupied by a friendly character, unless you are charging. When you move through a square occupied by a friendly character, that character doesn’t provide you with cover.
You can’t move through a square occupied by an opponent, unless the opponent is helpless. You can move through a square occupied by a helpless opponent without penalty. (Some creatures, particularly very large ones, may present an obstacle even when helpless. In such cases, each square you move through counts as 2 squares.)
If you want to try to sneak past the enemy linemen to sack the quarterback, you want the Overrun combat maneuver/special attack.