The goalkeeper is arguably the most important position on the field. A goalkeeper can win a game with one brilliant save or become the scapegoat with a mistimed dive. The nature of the position requires Technic, Tactical Knowledge, agility,
speed, athleticism, confidence, bravery and good communication skills. Communication skills so important for the keepers, there is a saying "It's not how many shot you saved it's how many shot prevented by communicating"
For decades, goalkeepers simply blended into the regular practice sessions with very little attention being provided to the goalkeepers specific needs. Goalkeepers are very different from outfield players and must be trained accordingly. Goal keepers do not get physically tired like field players but they get tired mentally, more then a field players. Specialized training is needed for specialized positions. Goalkeepers must be train to be mentally strong and able to deal with whatever comes their way. 
Goalkeepers deal with "WHAT IS" not "WHAT IF"

The role of the goalkeeper has changed more than any other position in recent years. With the introduction of the back-pass rule and changes of tactical side of the game, goalkeepers are now required to be as comfortable as outfield players with the ball at their feet. They touch the ball with their feet three times more then with their hands.
Goalkeepers are the last line of defense. They should think like sweeper backs and use the full range of the defensive third of the field. 
Goalkeepers are the first line of attack. Quality distribution is vital for counter-attacking. Keepers must possess a wide range of distribution skills in the area of kicking and throwing.
Communication between a goalkeeper and the team is critical for good organization. Whether it be setting up a defensive wall or calling for a high ball, the goalkeeper must be commanding, confident and clear. Bravery is a pre-requisite to play in goal at any level. Unlike outfield players, there are rarely second chances to win the ball in a one v one situation.

Coaching the Goalkeeper

Most of the coaches will stand on the halfway line or midfield to coach the defensive unit from out to in.
Coaches, should try sometimes to coach, through the goalkeeper position from in to out by standing at or behind the goal. Organize the defensive unit with goalkeeper, back line and even midfield.
This is a must for any coach to train/coach a goalkeeper. As the coach can feel, and see the field through their eyes and correct their positioning, tactical decisions more appropriately