He is understudying the famous film star.

他在为那位著名影星充当替角。

And I didn't even know that they were understudying it.

而且我以前甚至不知道他们当过预备演员。

But carbon fiber, which is used only sparingly now because of its cost and complexity, is understudying for a larger role.

由于碳纤维的高成本和复杂性，因此宝马对它的使用还比较谨慎，但它未来必然会在宝马汽车上扮演更大的角色。