Supernumerary: (of an actor) appearing on stage but not speaking.
The scene looks something like this:
http://youtu.be/xxgOIwOd_5I?t=55s
That is footage from an actual past performance at the Met!
If you fancy seeing the show, here are the remaining dates:
12/26 at 8pm
12/29 at 7:30pm
1/2 at 7:30pm
1/5 at 7:30pm
1/10 at 1pm
4/9 at 8pm
4/13 at 7:30pm
4/17 at 7:30pm
4/20 at 7:30pm