ON ACTING: 'Build'
Winners want to win; easily and early. So each character in a scene initially engages with a limited amount of energy in the beginning initial foray, or tactical effort of the scene. But when they don't win easily, quickly, cheaply against their adversary (the conflicual reality which creates a long scene), the actors-as-characters are forced to re-assess (this is sub-conscious, or at least sub-cognitive) before re-committing to the engagement with a greater effort.
And each time they re-assess (called a transition), and then subsequently re-commit (called a beat), they do so at increasing expenditures of effort/emotion/energy. They 'up the ante', as it were, playing each subsequent roll of the dice or playing of a card with increasing (and one could argue 'deeper' force); thereby ever increasing the character's emotional stakes, his/her expended intensity in the conflictual effort.
However, it is important to realize that a growth of intensity always occurs in the 'build' of an emotionally honest scene, but not necessarily the intensity's volume, pitch, pace; which may or may not increase, or diminish, or remain the same. It is the intensity that always increases in a 'build'. That's where the climax of the scene comes from: as in life, the increasing intensity--whether manifested loud or soft, high or low, fast or slow--finally gets relieved by the climactic resolution of the scene.