表示“……的'角色”可用“the role of ...”，一般不在其后面所接的名词前加冠词a/an。He will appear in the role of mediator.他会出庭，充当调停者的角色。
role用作名词的用法例句
The Panama Canal has played a very important role in transportation.巴拿马运河在运输上起到了非常重要的作用。
The key role of the teacher in the learning process should not be neglected.教师在学习中的关键作用是不该忽视的。
The role of the railway declines in the transport system.在运输系统中,铁路的重要性逐渐下降。