The hook is the nucleus of both a film and its screenplay. It is what grabs the viewer's attention, preferably in the first 5–10 minutes, as a reader might expect to find a literary hook in the first chapter of a novel.
During the pitch process, a screenwriter will use a hook to prove the "bankable" quality of their screenplay.
Knowing the importance of a good hook, many screenwriters write their hooks first. Conceivably, the life of a screenplay might evolve from hook to 1-page synopsis, to 4-page treatment, to full treatment, to scriptment, to screenplay.
One can briefly state a good hook in one or two sentences, introducing the protagonist, the conflict that drives the story, and what the protagonist will achieve with either triumph or defeat. The "hook" is the viewer's own question of whether the conflict can be resolved, so a screenwriter might want to test the hook by turning it into a question. For example, "Johnny must catch the murderer so that he can get the girl" might become "Will Johnny catch the murderer? Or will he lose the girl?" In this way, the screenwriter can use the hook as a tool when writing the screenplay.