Story structure
Rising action
The segment between the exposition and the climax where complications pile up and tension builds toward the peak.
Rising action is the stretch of a dramatic or literary work between the exposition and the climax. In Freytag's pyramid, "an exciting force begins immediately after the exposition (introduction), building the rising action" that carries the story up to the climax. In three-act terms, the second act "depicts the protagonist's attempt to resolve the problem initiated by the first turning point," with each attempt landing the protagonist "in ever worsening situations."
That last phrase is the whole job: the rising action must complicate. Attempts fail or cost something, stakes climb and suspense accumulates; if they do not, the middle goes slack. The midpoint splits the rising action into its reacting and acting halves, and what it builds toward is the climax, after which comes the denouement.