We have a commercial type popcorn machine with a 12oz kettle (great northern from Amazon). We purchase the proportioned popcorn from gold medal. It is a HUGE hit and we always get compliments on it.
Depending on when sunset is, we will either incorporate dinner in to the potluck concession stand (aka 2 6' folding tables) or keep it a potluck dessert concession table. Whichever the case, we ask the guests to bring something for the table and a 2 liter. We supply the plates, plastic utensils, napkins, cups and ice.
Our pre show entertainment has been trivia recently. There is normally a 10 minute break before the movie starts; this is when we start popping the popcorn. We don't have a warmer for bagged popcorn, so my wife scoops and bags the popcorn for each person.
I recently found some coated popcorn bags on Amazon. 1oz size I believe. I haven't had a chance to use them yet. But will be perfect for the butter-like topping I have.
I'm considering selling movie theater candy as a way to offset the cost of maintenance and upgrades. I haven't convinced myself that it'd be a success with the potluck concession table.