in the theater 在剧院工作
例句：
She is been in the theater all her life.她一辈子搞戏剧。
In the theater of celebrity tragedy,each play has three acts.
在上演名流悲剧的戏院里,每一出戏都有三幕.
at the theater在剧院看演出
例句：
Before he left Germany,a Jewish friend had given Christophe tickets for a box atthe theater.
在他离开德国之前,一个犹太朋友曾经给了克利斯朵夫几张戏院的包厢票子.
He testified that he was at the theater at the time of the murder.
他作证声称凶杀案发生时自己正在剧院
都可以，如果在歌剧院是in the theater 如果表示看歌剧，就用at the theater
