persona, character, function, office, purpose, use, part, theatrical role, responsibility, position, more...

bystander, spectator, observer, nonparticipant, outsider

lead, supporting, character, extras, stunt double, understudy, more...

important, major, significant, central, key, active, crucial, leading, vital, critical, prominent

part to play,   job position,   played a part,   play part,   what "it" plays,   part in a play,   part for an actor,   actors part,   casting requirement,   my part