Give a master carpenter a truckload of tools and a bunch of wood; he’ll build something. But hand him structural blueprints, the end result will be amazing.
Screenwriters work the same way – the outline is your blueprint. Never write without a plan; that’s screenwriting suicide. An outline gets you thinking (and all writing IS thinking), but it also keeps you focused and on track.
But how do you design that prefect outline? First off, nothing is universally perfect. Some writers put together comprehensive 20 page point outlines, plotting in every scene, even tossing in lines of potential dialogue. For other writers, breaking down the broad strokes of the eight sequences and making sure there is a clear central obstacle within each sequence is enough. But still others simply clarify the five major plot points.
Part of discovering the perfect outline, however, is trying different approaches and using what works best for you as well as for the script you are writing at the time. The more formulaic the genre (e.g. rom-coms), the more useful a detailed outline can be. An indie drama, on the other hand, may breathe better with more flexibility in the plan.
Regardless the many variables, however, I would argue that the most rudimentary outline must include these FIVE CORE ELEMENTS: the protagonist and his or her goal, (2) the supporting cast and what they each want, (3) the beginning and end, (4) the five major plot points, and (5) the order of events, sequences, and act divisions.