PIXAR’S 22 RULES OF STORY (RULE #3)

RULE #3: Trying for theme is important, but you won’t see what the story is actually about til you’re at the end of it. Now rewrite. 
I wholeheartedly agree that writers should write all the way to the end and then rewrite. In fact, I’d recommend doing that more than once. As the common aphorism “all writing is rewriting” points out, that’s the only way to really find your story.  
But as for not seeing what the story is actually about (its theme) until you’re at the end of it — I take the opposite tack. I don’t think you should even start the story until you know what the end is, therefore what it’s about.  
“What it’s about” will likely change during the course of writing a draft, but it’s too common to meander and write yourself into corners when trying to get to an unspecified ending.  
So if you don’t know how your story ends when you start writing, be prepared to pay a lot extra to get there.  

It’s like any journey you start without knowing where you’re going: it may be exhilarating and full of possibilities, the detours and pit-stops may be an adventure, and the end result may be fantastic — but it’s not efficient, and there’s a very real possibility you’ll get hopelessly lost and simply give up along the way.  
Starting at the end when creating your outline (or treatment, or mental map) will make your life a lot easier. And don’t worry that starting with a solid idea of where you’re going will stifle your creativity and take all the joy and inspiration out of the journey. It won’t. 
For one thing, until you’ve actually written at least one draft everything is still just preliminary, theoretical. While you’re writing towards an ending you’ve already come up with, you may suddenly find that the story is telling you to go elsewhere.  
That can happen even when you know where you’re going because knowing where you’re going is not a barrier to inspiration, rather it makes room for more inspiration
because there’s no need to be constantly “figuring it out” at every turn. So when this sort of inspiration strikes, stop and take the time to rework your ending, and the map to get there, before continuing. You can always go back to the old map if needed. 
Even after you’ve done all that, you are likely to reach the (potentially shifting) ending only to discover that in getting there you’ve got a whole new or clearer idea of what the story is actually about, and therefore about how everything you just wrote should change.  
That’s why people say all those things like: “writers write” and “all writing is rewriting” and “stories are never finished, they’re just abandoned” and “holy @#%! writing is difficult — I thought you just typed-in every idea you have as you have them and then people love you and throw money at you”.  
So, ultimately, what this advice is trying to tell us is don’t get bogged down in theoretical analysis of theme in lieu of actually writing the story.  
This is an especially damning temptation for screenwriters because screenplays are very structured and formal, and there is a glut of gurus out there who peddle very mechanical, theory-based approaches to storytelling.  
Those prescriptive methodologies can be great if you find one that actually works the same way your own mind works, but even so formal exercises about finding your theme (or character, beats, or anything else) will only ever take you so far.  
After all the end product is the actual writing, not any of the notes, outlines or worksheets produced along the way.  
Belaboring how each scene reflects theme and trying to perfect it is wasting time, especially in the first draft when you’ve not yet written through the piece at least once and thereby given yourself a firmer idea of what your story is actually all about. Once you’ve written at least one draft you can start to “perfect” all those beats, through- lines, and setup/payoff moments in rewrites.  
Ultimately, storytelling is about feeling, and even once you find your theme and refine your story structure you still need to make your audience feel it. 
So whatever methodology you may prefer for finding theme and structure, make sure that for each draft you also set all the formalities aside do a pass where you focus solely on emotion and entertainment.