役を務める (fulfill a role)

The set collocation 役を務める / 役をつとめる ('to play / serve in / fulfill a role'), built from 役 ('role, part') + 務める ('to fulfill, serve as'). Used for ceremonial parts (bridesmaid, host, MC), theatrical roles, and figurative positions in organizations or relationships. The verb 務める emphasizes serving in a duty rather than performing.