actor是可数名词,可用不定冠词或定冠词修饰。

He used to be an actor.他曾当过演员。
He used to be one of the famous actors.他曾是著名的演员之一。
That famous actor seemed content to play a minor part in the play.那名演员似乎很满意在剧中演一个不重要的角色。