Obstacles

If the protagonist and his objective constitute the first two important elements in the construction of a story, the various obstacles collectively constitute the third. 

You must put obstacles in the characters way, take care of the complications, and force them to find more inventive ways of coping with the situations. Only then, do you begin to see all the possibilities for flourishing scenes, because a scene is good only when it’s difficult, weary, and demanding for the character. If you don’t put obstacles in the scene, it’s flat, and it doesn’t offer anything to the others. 

Although the unity of the story depends on there being but one main objective, there is no threat to unity from the use of multiple obstacles to the achievement of that objective. 

In a film, when all the obstacles have been eliminated except the one that is most difficult for our protagonist, the audience is completely focused on an either/or situation. They will hope. They will fear. They will be engaged.