This is the meat and potatoes portion... AND the most difficult part of writing a screenplay. Most mediocre and flawed screenplays are plagued by slow or meandering second acts. Keep in mind the main tension - and that your character should always be on the path to resolving that tension.
The second act begins right after the lock-in: the moment when the character is stuck in the predicament and main tension - it is too late to turn back, so he/she must go forward. Now the character aims towards the goal, the objective, and he/she has the first meeting of the obstacles and antagonists or circumstances, always with rising actions.
The first sequence usually presents the alternative solutions. What are the choices? What should be done? And the character selects one alternative, and if it should be the worst one, then he selects another one, and in the meantime, the rest of the alternatives are eliminated. Then the character uses one of the ways to solve the predicament, and it seems to work, and that’s usually the first culmination or midpoint.
But it’s not that simple, because there are consequences of things that happened before that he didn’t take into consideration. He offended somebody. He didn’t do things that he was supposed to do. He forgot about things. You bring those things back in the second part of the act, and at that time they can be entered almost without motivation, because anything that works against your character at that time is acceptable. Any accident, any coincidence is fine because it makes his predicament worse, and therefore we enjoy it. Also it helps to explore the validity of the desire of the dream.
ACT TWO: ESSENTIAL ELEMENTS
OBSTACLES: The second act is all about obstacles. It elaborates in great detail and intensity on the difficulties and obstacles the hero faces as he or she struggles to achieve his or her goal. Just when we think the situation can’t get worse, it does. And when there is no way that our hero can get out of the jam, he does, only to end up in a worse jam. Basically, each sequence centers around a new obstacle or obstacles of increasing difficulty.
FIRST ATTEMPTS: Now that the character is locked-in, he makes his first attempts to solve the problem. This is usually the simplest, easiest manner to resolve the problem, and these attempts usually fail.
RAMIFICATIONS: It is important to show the ramification of the attempts, which must result in an increase in complications.
ACTION: Every move the character makes traps him even more. Each actions leads to more entrapment. Be merciless on the character.
SUB-PLOT: A solid sub-plot that carefully intertwines with the main tension in both plot and theme will be a great channel in which to relate the character’s emotions (in regards to the main tension).
FIRST CULMINATION: This is the midpoint of the film. If our hero is to win in the end, this then is the first time the character finds a solution that seems to work. It is a victory. If the script is a tragedy, however, this often a low point for the character.
MIDPOINT MIRROR: The first culmination and ending of the film usually mirror each other: both victories or both failures.
MIDPOINT CONTRAST: The first culmination (Midpoint) and the main culmination (End of Act II) are usually in contrast with each other.
NEW ATTEMPTS: By the first culmination, our hero has failed in his/her first attempts but in failure, realizes the weight of the issue and becomes aware of the correct method in which to resolve the main tension. He/she then can begin new attempts, still faced with new obstacles, that get him/her closer to resolving the issue.
CHARACTER CHANGE: Throughout the second act, the main character starts changing, learning, and developing, or at least intense pressure is put on the character to change, and that change will manifest in the third act.
MAIN CULMINATION: this is the end of the second act and the point where the character sees what he things he has been doing is not what he has been doing. The tension is at the highest point, and this is the decisive turning point. You must convince the audience that their worst fears are going to come true. This moment will change the main character in some way.
FIRST RESOLUTION: This resolution of the second act tension often spins the character(s) into the third act. (Luke Skywalker and Han Solo rescue Princess Leia from the clutches of evil Vader... but they still have to destroy the Death Star.)