Moment of Truth

Right at the end of Act Two is typically when our main character's situation is at its worst. All is in doubt, the chances of success look impossible, and the specter of failure rears its ugly head. It is at this Moment of Truth that the main character must face down their own inner demons in order to confront the actual demons waiting for them in the third act. This Moment is structurally crucial, because it sets up the consequences that lead to the final conflict and climax in Act Three.

Following are several Moments, and the irreversible Consequences they lead to.

"Alien"
The Moment: The remaining crew decide to self-destruct the ship, and escape in the shuttle.
Consequence: Ripley is the only one to survive the get to the escape shuttle, but she unknowingly launches with the alien aboard.

"Clockwork Orange"
The Moment: Alex, out of prison, has been rejected by his parents and beaten by his old friends, who are now ironically police.
Consequence: Alex seeks aid at the very house owned by a writer who he and his friends previously assaulted, and whose wife they raped.

"Jerry Maguire"
The Moment: Jerry argues with Tidwell, telling him he plays with his head and not with his heart, and Tidwell tells Jerry he doesn't want to be friends anymore.
Consequence: Tidwell is briefly tempted by an offer from Bob Sugar.
Subplot Moment: Jerry is unsure of his love for Dorothy, and they separate.
Consequence: Jerry and Dorothy feel alone without each other.

As "Alien" is primarily an action-based horror story, its Moment centers around specific physical action and its result.

In "Clockwork Orange," a morality play, the Moment is about Alex receiving the moral consequences of his actions.

"Jerry Maguire" is really two intertwining stories. The plot is a story about trying to have heart in a heartless world, and has a Moment defined by loss of faith. The subplot, a romantic comedy, has a Moment centered on the confused feelings of two lovers.

Understanding and analyzing these Three Keys to Act Two, then applying what you learn, will lead to greater cohesion and control in your own work. A greater understanding of how these structures are applied in successful films will allow you to use or depart from them as your story requires.