How do I replicate the taste of prepopped movie theater popcorn? To clarify, I'm not asking how to make popcorn that tastes like movie theater popcorn (which typically comes from a popper), I want to know how to make the bad pre-popped popcorn that small theaters sometimes sell.

It's hard to describe the flavor, but it was almost this hint-of-mustard salty taste with semi-stale, but large and uncannily round kernels. The closest I've found (and it is close) is the plain popcorn that comes in those tins. My guess is that there's a preservative used that's imparting a taste. 

Any ideas?
There's a different variety of popcorn used for pre-made popcorn, as it comes out more spherical and thus less prone to breakage once it dries out.  I've always seen it called 'mushroom popcorn'.

I've never made it specifically, so I don't know if the flavor is something to do with the variety of popcorn used, or if it's something added to the popcorn before storage.