But they love each other," Clary said, appalled. "Isn't that what love means? That you're supposed to be there for the other person to turn to, no matter what?" Luke looked toward the river, at the dark water moving slowly under the light of the autumn moon. "Sometimes, Clary," he said, "love just isn't enough.
A film is a living thing. The screenplay is a guideline. You really need to have a good, sound script to know that you have a dramatic structure that's going to work thematically, and to know how one scene will got through another, and to get a sense of character.