The first act sets up the conflict to come from seeds that develop and blossom in the second and third acts. The third act presents conflict resolution. The second act inherently has character development, amplifies conflict, and heightens tension. It additionally provides the reader who is going to buy the screenplay, and the audience, with a reason to become involved and continue with the story. Once writers understand the principles behind dramatic structure, including comedic structure, and work within and master structure, they can a break out and become less traditional in their structure, creating a non linear story or an existential tale.
In the 3 act structure, the main part of the story is told in the second act. Virtually every Hitchcock film follows this paradigm completely. The second act involves suspicion, miss cues, anxieties, game playing and all the other elements comprising the dramatic, romantic or comedy genres.
You have to know why you are writing the story. Logic create the worlds within which are characters live. Characters are the most important we must understand who they are, what motivates them, and where they are headed. Every one of these elements must find its way into the center of the story and pull a the audience with it. Without a strong setup the second act will fall apart or worse yet lose steam because there is nothing holding the action together, to compel the characters forward, or hold the audiences interest.
The second act is ever evolving turmoil that develops with a major crisis or several crises, expanding the world in which the characters live. The risks they face are physical emotional and moral. And although act 1 sets up the characters and their surface motivation, the second act develops the subtext and the characters and we have to provide the tools for the audience to understand look who and why of the characters.
Assuming we start with a good story and a set up a good characters who pushed the story forward then the second act becomes the arena where our protagonist can really operate and strive to reach his goal order reward. The reward can be monetary or it could be intrinsic and intangible. The second act is the core of the story where conflict and tension develop to a high degree. The refinement of tension becomes the center of the creative act and only if the writer has planted the proper seeds in act one and nurtured them in act two will it work.