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, careful. That's my original series Battlestar Galactica flight suit. Oh, why didn't you wear it on Halloween? Because it's not a costume. It's a flight suit. Okay, all right. Moving on. Oh, wow, a paisley shirt. Uh-huh, it goes with my corduroy suit. If you mean it should end up (..)2008 TvShow: The Big Bang Theory Title: The Cooper-Hofstadter Polarization Season: 1 Episode 9