The Act Two turning point is what Aristotle described in On Poetics as the peripety, the moment when the hero learns how bad things truly are. Their resolve is truly tested, and usually found wanting. It is when Oedipus realizes that he is the king killer he has been looking for.
In the usual three-act structure story, this is the moment when it appears that all is lost. It usually comes about three-quarters of the way through the film. It’s when Bruce Wayne is knocked unconscious as his mansion burns in Batman Begins, when Tony Stark struggles to survive after the theft of his artificial heart in Iron Man, the best friends are no longer speaking to one another in The Wedding Crashers, Hannibal Lechter escapes prison in Silence of the Lambs and the only inmate who can prove Andy’s innocence dies in The Shawshank Redemption. This is when it seems like the hero is beaten and that the movie is about five minutes from ending with the villain triumphant.
There is usually a glimmer of hope at the very beginning of the third act. If that comes in the same scene in which the protagonist came to the conclusion that all was doomed, it could to be difficult to figure out whether particular moments are from the end of the second act, or the beginning of the third act.
This was a slight problem when I was trying to determine the structure of Names Engraved in Marble, an Estonian war film. There was a scene before the final act in which the main characters were badly outnumbered, facing a rival army, and resigning themselves to the probability that the remainder of their lives could be measured in minutes. And then the cavalry literally came to their rescue. Whether the rescue occurred at the end of the second act, or the beginning of the third act is ultimately a distinction without any meaning, as there was still a moment of ruin roughly three-quarters of the way through the film.
In rare cases, the Act Two climax will be triumphant. This is especially possible if the film is going to end on a down note. So Bonnie may realize that she wants family and stability, Isla confesses that she still loves Rick, Harry Potter may get a reward at the Triwizard tournament, and the squad in Full Metal Jacket may achieve a military success. H/T Sciptlab