They were all in on the scheme.他们全都参与了那场阴谋。

The characters are convinced that they're on another planet, but the audience is in on the joke. [=the audience knows they are not on another planet]这些剧中人物坚信他们是在另一个星球上，但观众知道这只是个玩笑。

I let them in on our little secret. [=I told them our secret]我把我们的小秘密告诉了他们。