It depends upon the position. For QB, K, and P, the number one player will most likely be the only player to get any playing time. For the other positions, it depends upon the formation being used. For example, if your team is in a 4-4 formation on defense for a play, then the top four defensive linemen, top four linebackers, and top three defensive backs will be in the game for that play. The player listed first will be placed in the most critical position. This means that the first running back will get most of the carries and the first defensive back will cover the other team's number one receiver, etc. Of course, injuries and game fatique will cause some adjustments. You can have more control over this aspect of the game by using the advanced depth charts.