How a player behaves during a match depends for a large part on the lineup position you assigned him to.
The keeper usually stays in his goal. He may move to the penalty area when he has the ball to be able to pass it further upfield. He may also perform tackles to try to capture the ball from an attacker, but this won't happen often.
Central defenders are the last players between your keeper and the opponents attackers. They will usually move to the penalty area when the opponent is attacking to be able to block shots from the forwards. They will often try to capture the ball from opponents using their dueling skill.
When attacking, the center backs move forward further than defensive center backs. Defensive center backs never go forward when their team is given a corner, center backs may do so.
These defenders have a scoring bonus of 1.5 ball. This may be very interesting in attacking corner situations.
Central defenders need good blocking and dueling skills. They will be on the receiving end of most of the opponent's attacks so you'll want your best defensive players here.
Wing defenders are responsible for defense on one of your flanks. They also help in the attack, capturing the ball from the opponent and bringing it forward. LWB and RWB are more offensive than LB and RB, they will move further on the opponent's side.
LB and RB players also get a 1.5 ball scoring bonus, LWB and RWB only get half of that bonus.
These players need good dueling skills, but also benefit from tactics (to move to the center when needed). LWB and RWB players will regulary get into positions where they can give assists so they would also benefit from good passing.
The sweeper, or libero, is the last defender when the opponent is attacking. When in control of the ball he has a more free role and can advance as far forward as the opponent's penalty area.
Sweepers have 1 ball scoring bonus. A sweeper can be a very valuable player but only a special type of player can do well on this position.
Of all the midfielders, the defensive midfielder is most likely to attempt tackles. While other midfield roles are less likely to put pressure, the defensive midfielder has the task to stop attacks on the midfield and switch to attack quickly.
These players need dueling skills to capture balls and passing skills to be able to pass to forwards since they are often in a position to give assists.
Midfielders are the link between your defense and your forwards. Since the success of a pass depends on the strongest passing skill of the receiver and the sender of the pass, a good midfielder can successfully receive the ball from a defender and pass it on to a forward even if neither the defender or forward know a lot about passing. Offensive midfielders are less likely to attempt tackles and stay further forward during defense.
Passing is very important for these players, all the other skills can also be used in these positions. Blocking to intercept passes, dueling to capture balls, scoring to take advantage of weak defenses and tactics to outmove their opponents.
Forwards need to score goals, they don't often attempt tackles and usually stay on the opponent's half, even when the team is defending.
The LF and RF players are more often in a position to attempt distance shots. This will be especially effective against teams with weak wing defenders. To do this, such players would benefit a lot from a high power.
Forwards need scoring, a lot of scoring. They can use dueling to distract defenders, giving another forward a better chance to score. They can also use passing if you want them to be able to give assists.
Any player who scored a goal will receive 0.5 ball scoring penalty. A second goal in the same match will give an additional ball penalty, the third goal 1.5 ball etc. This makes it harder for one forward to score a lot of goals.
A similar penalty exists for midfielders who get a reduction in passing for each assist. This reduction is only half as big as the reduction in scoring.
You will notice that players won't always strictly follow their orders. This can have several reasons.
A player with the special attribute "roaming" will often leave his assigned position. He will return to it on standard situations (corners, free kicks) or when play is resumed after a goal. The roaming attribute has no effect for keepers.
Players with low tactics are also less good at following orders. They will more often move unexpectedly. These players will also more often make bad decisions when deciding which player to pass the ball to.