他在为那位著名影星充当替角.He is understudying the famous film star.主角演员病了, 让他的替角出场.The leading player is ill. Let his understudy to appear on the scene.主角演员患病,由替角代替演出.The understudy was substituted when the leading actor fell ill....

“替角”的英语可以翻译为：understudy ...