First, terminology. You don't buy a movie stock to short it. That implies buying it long, selling it, then shorting it. You can just say shorting it.
Second, it is always best to have an idea of your goal when you enter a position, i.e. short it. Do you think it will go down significantly over the long term, or are you shorting it for a quick drop? Only you can answer that question, which you should be asking yourself regularly.
Three, there are many strategies to shorting, and it is a great way to grow your port. Some hold for the long haul, as many moviestocks simply decline on a long term basis. Some short after a big pop in the stock, for short term gains. Some short an overpriced moviestock that won't delist at its current price. Come up with your strategy and have fun shorting!