Movie makers are master illusionists. In China, the boom in the movie industry could be a grand illusion.

Box-office receipts during the Mid-Autumn Festival, a three-day holiday this month, fell 15% compared with last year, despite moviegoers having an additional day off this year. The poor showing follows a similarly disappointing summer. Box-office receipts from June to August fell 4% from a year ago, according to Deutsche Bank. This is an abrupt change of sentiment from just the turn of the year. Goldman Sachs said in January...