You Can't Plant Seeds in July
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.