People that are buying and selling the stock.Ā The weekend that the movie opens, the stock is halted by HSX (so no buying and selling it once it halts).
When the weekend box office ESTIMATES come out by the studios.. this is reported by ERC - which is the 'official' figures that HSX uses to adjust the stock price when it's unhalted... HOWEVER, these are just ESTIMATES from the studios, and even though HSX uses these estimates for the adjust price, it does not meant that it is EXACTLY what a movie made at the box office over the weekend..it's just a close estimate.
Once a movie stock is UN halted on Monday (night) then the price of the movie stock is again subject (ONLY) to people buying and selling it.
The closer the movie gets to delisting (after 4 weekends) the closer (usually) the price of the stock gets to the actual box office earnings.
The ONLY time a movie stock is NOT subject to market forces is when it is halted and adjusted in price, any other time during its 4 week run, it's people buying and selling it that affects the price, UNTIL it is delisted at the actual box office earnings.