If you wait until you're almost done writing your script to decide your protagonist's main flaw, then I hope you enjoy cutting your script into a hundred pieces, searching for the right places to plant those seeds.
If your husband never expresses to you that he hates how you fold his underwear, and then one day he unleashes a wrath of epic laundry proportions, then you'll be a little caught off guard. That's not fair to you, right?
If you do the same thing to your audience, it's not fair to them. In fact, they won't even stay to fight with you, they'll just walk out on you, pissed off and annoyed.
You have to plant the seeds early on in your story, so when something happens later, whether it be the explosion of anger at the 75% mark, or a simple choice to have a few extra drinks in that restaurant scene, your audience has been prepped. Whether they realize it, or not, the audience wants to understand your protagonist. The better the audience knows the protagonist, the more invested they are in the choices your character makes. Plant a seed on page 3 that your protagonist is particular with how people do things, or plant a seed on page 12 that your protagonist has a fear of commitment.
Even Ferris Bueller's Day Off, which has one of the weakest arcs of any protagonist (awesome character, but weak arc) has seeds planted throughout the story to warrant what happens later.
Ferris is babied by his parents in the beginning, which helps us understand how easy it is for Ferris to get away with what he does. They are almost unwilling to doubt his trustworthiness later on, even though his father spots him in Chicago, and his mother finds a machine sleeping in his bed rather than him.
Ferris selfishly bullies Cameron into taking his father's car. His naïveté towards the effect he'll ultimately have on Cameron gives Ferris more depth when he witnesses Cameron's cathartic moment at the 75% mark.
And technologically speaking, we see in the beginning that Ferris has a knack for manipulating his computer (his consolation gift when Jeanie got a car.) Later we see how he uses his computer skills to build a snoring dummy to fool his parents (but obvs. not Jeanie.)
Plan ahead. Figure out what your protagonist is about before you start writing, so you don't have to deconstruct them (and dismantle your entire script) once it's too late.
And if you want strawberries in July, plant the seeds about 10-12 weeks before the last spring frost.