I wrote this as a response to a post on G+, I really want to write more about this because it aims in the same direction as a lot of my Walkabout work. But I'll just leave this here for now...
There's a form of literary analysis common used for analysing noir and crime stories. I can't remember what it's called (it might possibly be "russian formalism").
If uses the concept that an investigative piece actually has two stories, the "subject" and the "fable". The subject is the story unfolding _now_, while the fable is the story being revealed...the story that has already occurred. The Subject is capable of changing with the decisions of the protagonist(s), while the fable remains a fixed point.
Pulling this back into gaming, I like to write a vague fable in point forms. I leave the subject purely open to improvisational play, the characters in the story can go wherever they want to reveal aspects of the fable. And since the fable is only key points, it has enough blank gaps to allow players to throw in their own hypotheses. The fable isn't written in stone (it can react to the ideas of the players), but it's events have occurred (the characters can't manipulate it in any way...only reveal it).