How do you know what the story will be?

I've been watching a lot of how-to D&D content lately on YouTube and I cannot believe how many people are fixated on "telling their story" or "making compelling characters" for when they sit down to play.

How can you tell what your story is going to be before you get together and make it? 

How can you tell who your character is going to be before he shows you through play?

Story is what you get out at the end, not what you put in at the beginning.

Admittedly this is me being obtuse and pedantic. Some people like story paths. Some people like railroady stuff where the players figure out the "how" of the DM's plot rather than the "what" and "where" of the PC's ideas. 

This is literally my play style summed up in one photograph

But to me, there's nothing more satisfying than telling the story of a completely unexpected adventure that materializes when you all least expect it. And there are ways to prompt and foster that amazing experience and, I suppose, procedurally generate such a tale. 

But the question is, what are those procedures that will generate that unexpected story?

Fallen Empire is a setting (not a product - it's nowheres near complete) that is attempting to put those procedures and elements in place. 

There's only a hint of story: "Here's A. What B do you want to get to, and how do you plan to do it?" along with some shared assumptions. 

Along with the several resourced I've made for myself and collected from other people, I expect to have a good shot at answering all the "what ifs" that come up between A and Z.

I wrote an essay a few years ago about this. It said just about the same thing I'm saying now but in a different style. At some point I should share it here.