Dramatic effect derives from what is probable, and not from what is possible. - Aristotle
The course of events the screenwriter sets in motion has not only followed the plausible path; the audience comes to believe there could not possibly have been any other outcome. This feeling of inevitability - a combination of characters moving along a course from which there is no possible turning - is perhaps a screenwriter’s finest achievement.
Inevitability is the sense, as the events unfold, that they couldn’t have happened another way, while predictability relates to the audience’s capacity to guess what is about to happen. So long as there are two equally plausible outcomes preventing the audience from guessing what is going to happen in the next scene or sequence or at the resolutions, the story is not predictable. And if, at the same time, each step along the journey or the story feels probable and the hand of neither God nor the writer is visible, the story’s unfolding events will seem inevitable.