Disclaimer: I play systemless narrative games.
Most, if not all, the games I run follow the same structure: There is a cast of NPCs and factions that have certain goals. Clearly, this generates a conflict of some kind. As a GM, I know what the various NPCs can do, how they react, and how they behave. If there were no players, I cancould tell who would achieve their goal, how they would deal with the opposition, and how events willwould play out.
The PCs interact with some aspect of said conflict(s). From there on, there is no plan. Whatever the players do influences, changes, and/or morphs the NPCs/Factions. So, after each player action (technically, player character action), I (as the GM) have to work out how the world will react to their action.
In addition, I always heavily link the player characters to some factions/NPCs. This allow the player to know that their background was used to write the game. It tightly couples PCs to the (sub)plots. Thus, if a payer wants a certain story arc for their character, they will get this.
The difference between this approach and a book/script is simple: the players get to decide where the plot goes and how the conflict(s) get resolved. It is a collaborative effect where what happens is cast in stone and is cannoncanon. You cannot retroactively change it.
Could we all write a book/script instead of playing the game? The no retroactive change allowed make this difficult. Without it, it could lead to something like Good Omens... If the participants were Gaiman and Pratchett. ^_~