It is the event, piece of action, perhaps but not often, the piece of dialog that happens that grabs your audience’s attention; that “hooks” them into the story and makes them want to stick around to see how everything turns out.
In the best of screenwriting worlds, a story’s hook should appear within the first five to ten minutes of the film.
In screenwriting terms that means no later than page ten, one page equaling one minute of film time following proper screenplay format.
The hook can be something traumatic, or surprising, or confrontational, or shocking, or embarrassing, or weird, or unexpected, or whimsical, or downright fantastical as long as it’s written so that your audience can believe it, and so that they can buy that it’s feasible.
It also has to be absolutely essential to the story.
If your hook doesn’t fulfill those criteria, you’ve already lost your audience’s suspension of disbelief and their interest in the film.