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'.
(..) - Doesn't he look wonderful? - Absolutely. I'm so proud of you, my boy. Oh, dear. I sometimes wonder if this is all down to me, making you those costumes for your little plays when you were tiny. - You used to adore all that dressing up. - Nathalie... Does it still make you feel special, Ralf dear? (..)2008 The Boy in the Striped Pajamas