6.1 with 5.1 movies?

My Yamaha AV receiver is 6.1 capable(Dolby Dig EX). I currently don't have a rear center speaker. If I get one and I play a 5.1 movie will any sound come out of my rear center channel?
Will it be matrixed or simply not play at all?
My guess is that it can probably be either, depending on your setting. If I'm not mistaken, Gladiator is both 6.1 and 5.1. Get that, then you could compare.

What receiver do you have? On my RXV-1000 their is a phantom ex mode that matrixes the rear center channel from the two other rears. You have to turn it on though. On my material encoded for ex or es surround you can set up the receiver to detect it automatically. If you want to have a discrete rear center channel you can hook up a pro logic receiver to power the rear center.
I have the RXV-630. I found the setting to switch it to AUTO or 6.1matrix. Thanks for you help.
I own an EAD ovation-8 and most movies I watch are 5.1.I like the rear ch. to fill in inbetween the rear surrounds.It must be matrixed if the movie is 5.1. I use just one rear ch.(room size only permits one ).Good luck