“替角”的英语可以翻译为：understudy ...

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....