Paradoxically, Act III writes Act I. The beats of a movie’s beginning should be crafted by knowing where the protagonist will end up. Act I can only be (re)written successfully by imagining the character at the end of the movie and then rewinding his life back to a place that is, emotionally, far enough away from that end to launch a meaningful internal journey.
One of my favorite screenplays is Kramer vs. Kramer (1979) by Robert Benton. It’s a perfect example of Act III writing Act I. Think of the contrasting French toast scenes: Disaster in Act I. Touchingly synchronized in Act III. Think of the ways in which Ted drops Billy off at school: Ignoring his son, handing him off to someone else and rushing away in Act I. And then, in Act III, parenting firmly (he insists on a twice-weekly hair-washing for Billy) and gently (he kisses him and calls him a terrific kid before sending him inside).
The whole focus of the film is Ted’s development as a parent, so the successful fathering of Act III must be set up by the failures of Act I.
It might be tempting to think that the scenes of Act I led to the ideas for Act III. But the only way to know what to make Ted fail at in the film’s beginning is to know what he will succeed at in the end.
Act III writes Act I.