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'.
(..) Oh, no. Oh, no. Make way for the fastest man alive! Oh, no. See, this is why I wanted to have a costume meeting. We all have other costumes: We can change. Or we could walk right behind each other all night and look like one person (..)2007 TvShow: The Big Bang Theory Title: The Middle Earth Paradigm Season: 1 Episode 6
(..) - Of course. - TV, film, D- and-D, manga, Greek gods, Roman gods, Norse gods-- Anything you want! Okay? Any costume you want. Bye. Gentlemen, to the sewing machines. Season Episode : The Middle Earth Paradigm I'll get it. (..)2007 TvShow: The Big Bang Theory Title: The Middle Earth Paradigm Season: 1 Episode 6