The dancers were dressed in their 'national costumes'. [=the clothing that is traditional in their countries]
If all the actors are 'in costume', we're ready to begin.
The waiters were all 'in' ancient Roman 'costume'.
(..) I've got to go to work, buddy. Oh, and another thing... If you're going to be staying here, You should think about, you know, Getting rid of the costume. But I've worn this my whole life. You're not in the North Pole any longer. You wanna make me happy, don't you? More than anything. (..)2003 Elf
(..) Hi. What do you want for Christmas? A Suzy-talks-a-lot. I'll put in a good word with the big man. Thanks. Your costume is pretty. Oh, it's not a costume. I'm an elf. Well, technically, I'm a human, But I was raised by elves. Oh, I'm a human raised by humans. (..)2003 Elf