Popcorn is my favorite! In induction, I was very good, didn't stray at all. The way I did this is with lots and lots of water, the same kind of snacks mentioned above, sunflower seeds, and trident sugar free bubble gum flavored gum (1 carb per piece).
However, if none of the suggestions so far seem to strike your fancy....I would suggest a small unbuttered. I know that Adkins allows you butter, however, most theaters don't use real butter, therefore....you run the risk of hidden carbs. The small popcorn will run you anywhere from 12g to 20g's of carbs.

If I remember correctly, induction suggests you stay between 20-25g's a day. One small popcorn could be the carbs for the day. That would only leave you meat as a carb free food. Like I said, I'm trying to remember. Induction was a long time ago.
Your choice.....good luck and enjoy your movie!