When I’m writing an outline for a new script, the first thing I do immediately after establishing what the protagonist’s external goal will be, is to figure out what could happen that will guarantee that the protagonist will fail at that goal. To put it another way, what is the worst possible thing that can happen in the protagonist’s pursuit of his external goal? To put it in yet another way (I promise I’ll stop rephrasing soon), what is the most devastating thing that the antagonist can do that will cause the protagonist to fail? Whatever the answer is to any of the three questions above, that is what should happen at the end of act two. It should appear at this second act break (about 75% of the way into your script) that the protagonist has failed at his external goal, so don’t go easy on him here or let him off the hook in any way. Do everything you possibly can to destroy all of his hopes and dreams so that the audience will be all the more impressed when you show how he overcomes those seemingly insurmountable obstacles and succeeds in the end.
The problem that I see too frequently is that the low point at the end of act two doesn’t have anything to do with the protagonist’s external goal. In one script that I read, the protagonist wanted to become a professional boxer, but then at the end of act two his mother died. That’s extremely sad, but it doesn’t do anything to prevent him from becoming a boxer. If you want your story to be tightly integrated and highly dramatic, then the low point at the end of act two must make the audience think that your protagonist is no longer able to achieve his external goal.
If you really want to show off, you should also make it seem like the protagonist can no longer overcome his internal flaw here as well. A common way for an antagonist to defeat the protagonist at the end of act two is for him to expose the protagonist’s internal flaw for the whole world to see. This usually causes the protagonist to slink back to his ordinary world in act three, an utterly defeated shell of a man, which is great dramatic stuff. If you shut down all hope for your protagonist both to accomplish his external goal and also to overcome his internal flaw at the end of act two, you will be miles ahead of at least 90% of all of the scripts sitting on agents’ desks right now. Just follow the old axiom that goes something like, “put your protagonist into a tree, then throw rocks at him, then disintegrate his entire world into a nuclear holocaust that will make Revelations seem like a children’s popup book” and you’ll be fine. I may be slightly misquoting there, but you get the point.