I think there's a common misconception that outlining is somehow divorced from actual writing, or that the goal of outlining is build a rigid plot skeleton that you then have to cram your story into no matter how much your characters and themes bulge grotesquely out the cracks.
On the contrary, I see outlining as simply the process of thinking your story through before committing it to its final format. Unless you have Jon Peters standing over your shoulder telling you Superman has to fight a giant spider in the third act, the point isn't to retrofit your story to hit predetermined points, but to be able to go through and examine from all angles how your story and your characters progress. Who are your characters? What is their situation? What happens that alters that situation? How do they react to it? What does that reaction drive them to do? How has that choice changed them, and what is the new situation they're faced with as the result? You could stop at the first question and start scripting, but what happens 60 pages in when you realize you took a wrong turn back on page 20 and your whole story is suffering from what seemed like the right choice at the time? If you're incredibly disciplined, maybe you go back and fix what needs to be fixed and redo 40 pages from scratch, but if you're anything like me, odds are you just get frustrated and put the whole thing to the side, hoping to return to it but never actually doing so.
To me (and this is the result of a major evolution over years of writing), outlining is just as much a part of the process as scripting and revising are. And the lovely thing of it is, the more you focus on the outlining, which can actually be fun and creative and gratifying, the less you typically have to worry about the revising, which tends to be mind-numbing and soul-crushing.
The process I go through now, which has made my writing so much better and so much more rewarding, is: start with a very basic description of your story, essentially your logline. If nothing else, you want a sense of what kicks off the story and the general direction that takes it. From there, figure out who your characters are. Beyond general description, you need to figure out what their internal struggle is. How is each person in your story affected by the events of the story? You absolutely positively HAVE TO know who your characters are before you start plotting, because everything that happens will derive from the actions they choose at each step of the way, and if those choices are not beholden to some logical consistency built into those characters, your story sucks and I don't want to see your movie.
From there, I write a fairly brief prose outline, maybe 2 to 4 pages, just working out the broad strokes of the story. This is where you really want to put the last step to use, because every time you ask yourself, "What happens next," the answer is always going to be found in your knowledge of the characters. And the deeper that knowledge goes, the easier those answers become clear. It's at this stage that the major themes of your story will likely start revealing themselves to you as well, and that's a fun process to be a part of.
That done, if you want to start writing the script from there, I would fully support your decision. For me, though, I would move on to the scene-by-scene outline, which is exactly what it sounds like. Using my previous outline as a loose guide, I start working out the setup and payoff of every single scene in the movie. Where are they, who's in the scene, what happens. There are two huge benefits to this: it will cut the time it takes to you finish the full script immeasurably, and it will help you find major problems and fix them before you've committed too much to paper.
I guarantee you, if you take the time to work through these steps, writing the actual script will be easier, faster, and you'll be so much happier with your first draft than you would be if you just started scripting with nothing more than an idea and a title (side note: starting with a title is fine, but 99% of the time the script itself will suggest a much better title once it's done). I find that when you've put this much thought into it beforehand, issues will become apparent immediately as you're writing, and solutions come much more easily. It's a wonderful feeling to hammer out a solution to a problem in your story and realize that same change solves a problem elsewhere in your story. I'm not saying that's impossible to achieve when you're writing by the seat of your pants, but it seems to happen more easily and more often the more thought and effort you've put into the story and the characters prior to the actual writing.
Aaaaand that concludes today's edition of Pretentious Thoughts on Screenwriting with Polkablues. Tune in next week for "'As I'm Sure You Know...': How to Better Hide Exposition in Dialogue."