A certain role may be task oriented, or situational. For example, a running back specializes in short-yardage situations or a defensive back plays during obvious passing downs. Injuries, fatigue, game circumstances, improvement in ability and expansion of versatility are all considerations that lead to a greater role or more roles.
Perhaps the most important roles on a team are more intangible. The needs of a team dictate the necessary intangible qualities, i.e., a team needs a vocal leader and a leader-by-example. Other possible intangible roles: a player who encourages his teammates as they enter or exit a game; a player who leads the pre-game warm-up; a player who knows the plays well enough to remind or counsel a teammate; a player who says little and listens more; a player who gauges the emotional in-game needs of his team and responds in an appropriate manner; a player who is a liaison between other players and the coaches; and so on ad infinitum.
It is the responsibility of each team member to discover his role(s). Often, the role is assigned by coaches. As often, a role is identified and fulfilled by the proactive player who searches for the myriad ways to help his team. It seems counterintuitive: the shortest path to individual greatness often lies within the maximization of tangible and intangible team roles. Be a great teammate; become a great man.
From October 2010, http://raising-a-man.tumblr.com