DRAMATIC principle is that apparently mysterious quality of a play, whether for the stage or the screen, which vitalizes it with an interest for the spectator or auditor holding him enthralled, uncertain of the results, sympathizing with the emotions and efforts of the characters, hoping and mentally fighting with them for the successful denouement, and ultimately satisfied with, or reconciled because of, certain natural laws of justice, to the completion of the action of the play.

The life-germ of dramatic principle is STRUGGLE.

The life-nourishment of dramatic principle iS HEART INTEREST.

The life-stimulant of dramatic principle is SUSPENSE.

The life-joy of dramatic principle is ULTIMATE JUSTIFICATION.

The above sounds pretty useful and screenwriters would be wise to focus on "Struggle;" which could also be described as conflict or tension.  The above advice was given by Eustace Hale Ball in 1915 during the silent film era in her book Photoplay Scenarios: How to Write and Sell Them, which you can read for free here.