Does popping kernels of corn in a brown bag actually work? YOU BETCHA! I put 2.5 tbs in a brown sack, fold, fold, fold, squeeze the fold and then microwave for 2 to 3 minutes. (Listen for the pops) No oil, no butter and today I topped it with feta and smoked paprika. For this popcorn junky this trick is a winner!
You can always use spray butter if you like. As a recovering butter-holic, spray butter seems to do the trick.