When a team is performing at its best, you'll usually find that each team member has clear responsibilities. Just as importantly, you'll see that every role needed to achieve the team's goal is being performed fully and well.
But often, despite clear roles and responsibilities, a team will fall short of its full potential.
How often does this happen in the teams you work with? Perhaps some team members don't complete what you expect them to do. Perhaps others are not quite flexible enough, so things "fall between the cracks." Maybe someone who is valued for their expert input fails to see the wider picture, and so misses out tasks or steps that others would expect. Or perhaps one team member becomes frustrated because he or she disagrees with the approach of another team member.
Dr Meredith Belbin studied team-work for many years, and he famously observed that people in teams tend to assume different "team roles." He defined a team role as "a tendency to behave, contribute and interrelate with others in a particular way" and named nine such team roles that underlie team success.https://www.mindtools.com/pages/article/newLDR_83.htm