The first thing we try to do is to find 'comprables' (other films that have similar features like - star, director, similar subject matter, rating, time of year released, etc) and then we blend in other factors (like - how has box office been doing lately, how has this type of movie been doing lately, other movies opening same weekend, competing movies, amount of advertising, quality of advertising, etc.)
slappydavid posted a great list of Tyler Perry movies. Looking at the list we see that Tyler Perry has been successful and has had to franchises with 'Madea' and 'Why Did I Get Married'. 'Confessions' is a Tyler Perry movie that is opening this weekend but is not a part of those two series of movies. So we see what else we can compare it to (on the Tyler Perry list). We could compare it to 'I Can Do Bad All By Myself' but that was released at a different time of the year (in September) so we keep looking... and we find that :
'Meet The Browns' was released on a 3/21 weekend and made $20 mill opening weekend
'Good Deeds' was released on a 2/24 weekend and made $15.5 mill opening weekend
Going by Tyler Perry's track record of movies released around this time of the year (and not counting his sequels) we are looking at a range of 15.5 - 20 mill opening weekend.
Next we compare our range to 'tracking' (companies who survey moviegoers interest in upcoming movies). RS (one tracking company) predicts "mid teens" (~ $15mill) opening weekend based on their surveys (link). MTC ("Major Theater Chain" - who does their own opening weekend predictions) predicts an $18 opening weekend (link).
The tracking is supporting our 15.5 - 20 mill opening weekend range (now sounding a little more like 15 - 18 mill opening weekend).
'Confession' subject matter is aiming for an older audience so we check and see how movies with similar subject matter have done. 'Obsessed' (link) opened near this time of the year, and had an African American cast dealing with martial issues and had a 28 mill opening weekend. So it is possible for 'Confession' to get a strong audience turnout. So let's pretend that that fact sways us to thinking 'Confession' will tip to the higher part of our 15-18 range and lets settle on 17 for now.
Let's look at the competiton. When 'Obsessed' opened it did not have any competition (similar themed movies - or any big movies to steal its audience)(link). 'Confession' is opening this weekend against GI Joe sequel and The Host. Neither movies are competing for 'Confession's audience (Gi Joe = Male; The Host = Teenaged Females). Confession is generally trying to target older females. So our 17 is still safe.
How has box office been doing lately? The year has started off slowly but just last weekend Olympus Has Fallen exceeded predictions, so box office is generally healthy right now.
I am not an older female so I can not gauge how the advertising has been. It is aiming at a certain audience (older females) so its advertising is most likely focused on TV shows, movies, magazines, etc also aimed at that demographic.
So after all that, we still have a base range of 15.5 - 20 with a likely 17 opening weekend, with a possibility of a higher opening weekend, but then we remember that Sunday is Easter and older women are likely to have/be involved in family events on Sunday and that takes away the chance of a breakaway hit, and we are left with 15.5 - 20 range and a target of 17.
Each movie, each star, each weekend, each competition varies each weekend, but the above is just a sample train of thought to get an estimate/prediction of how much a movie will make opening weekend. Some traders factor in theater count, rating, weather, pre sale tickets, twitter mentions, facebook likes, quality of poster, merchandising, etc..
Some players have/have had other strange factors that they mix in their thoughts... one dude factored in how many cars were in the parking lot as he drove by the theater on opening day, one creepy dude asked random teenaged girls in the theater lobby what they thought about upcoming movies and factored that in his thinking,... some students at MIT tried coming up with a computer program formula to predict box office, one dude factored in whether his dog liked the TV ads.
There was one trader that I noticed over a period of a couple of years ALWAYS got EVERY movie wrong from Halloween to New Year's each year. So one year (for fun) I just automatically invested the opposite way of his posted 'gutchecks' and got every movie right!
In short, that is what we are doing here at HSX... trying to figure out which stuff should be factored in and how much it could affect the opening weekend box office. Some of it is hard work (research), some is random guessing, some is nonsense... good luck!