Muppetstock is up today because some players decided to buy the stock ( or cover their shorts - but forget about that... it makes this newbie-explanation too difficult )
Whenever an individual player buys ( f.i. ) 100.000 stocks of a title, that stock gains a few cents in value ( 2 cents... 10 cents ... that depends on the value of one stock plus how many value they represent in total in the market )
When another player also decides to buy that stock, its'value increases more.
The same goeds for selling ( or shorting ) stock... but then the stock loses some of its'value.
That's it in short really.
The REAL value of a stock is decided the day after the weekend it has opened. And that is where your honest guess of the films performance lies ! That's where you can really let your money do the talking. Or as they say; put your (fake-)money where your mouth is. Offcourse you can allways make money on stockmovements in the days, weeks, months, years BEFORE it's release, but you can't let your own personal opinion/expectations of a film ( on whatever basis... trailers, rumours whatever ) count as important, true, professional, non-debatable or whatever else firm belief you might hold.
You can trade with a longview vision and ignore daily stockmovements ( that viewed over a month often show their real value/movement ) or daytrade in response to the daily movements.
In the last case; don't forget to sell those same stocks at the end of the day, because the next day they could be down.
When a stock in your portfolio has gained 3 % or more, you could ( Could !!! ) sell it and then you would have made a minimal profit ( transaction costs included )
As a rule I sell stocks I don't firmly believe in longterm', whenever they made 10 % or more.