Are you checking ads or trailers? Some product ads start before the posted "showtime," but those are usually just the pre-show ads they have while people filter in and out. I have found that the ads usually start at the showtime, followed by the movie trailers. The company I have done these for (which I assume is the same one you are doing) asks you to be there 20 minutes before the showtime, but I haven't found that to be necessary, if I am checking trailers. By the time the movie actually starts, I have been in the theater for about 20 minutes past the posted showtime. If you are doing trailers, I think you might be cutting it close. The trailers for the 5:05 showing might not be done before the 5:20 showtime, but the trailers for the 5:20 might not start right away.
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Could I have a receipt please?