My design of adventures consists of research and creation of the environ. I'm going to be prepared to talk about the places and people you are going to meet and try to influence.
Layered on this, I will note about one plot thread for every two players (two minimum) and then wait to see what the Players make of the stage.
Often I have "milestones" in mind. Sometimes this is surprisingly simple. From Argent Rose we had:
Yet if the ocean liner had been destroyed by a PC halfway across the Atlantic, we still would have had a game. Just not the game in the con blurb.
I expect Players to make choices suggested by their Characters and faithfulness to the story background. I don't expect a PC to switch to the Stigmatist side in the middle of an Argent Rose game. That's not being sincere to the background.
OTOH, it's perfectly reasonable for someone to try and join the Chaosian side in Bloody Grievance. No one really has tried that—and probably wouldn't—but it makes sense in the context of Amber genre. It is a choice I can't exclude as the game designer.
Then there are sticky choices.
Sticky PCs will not just go with the flow, they will demonstrate how the flow comes from them and is part of who they are. They take the background and other characters to heart. They make the game their own.
In Argent Rose, when one PC missed the ship leaving for America, William was sticky. He immediately made extraordinary efforts to accomplish the trip as planned by getting off the ship and finding the missing cousin—then catching the ship again at sea. This involved a quick parlay with the ship's captain, a powered launch back to the docks, racing through Paris to the palace, basement briefings, another race across Paris, and two stops to finally find the cousin at her lab.
Amber genre would certainly allow the PCs to sit around on the leaving ship and chat about how much trouble the missing Chloe was going to be in with the President of France. But that is much less sticky. The game was better for William's actions. It wasn't plotted that way—or expected—but that "left turn" also set up in-game jokes about "Chloe's late again" throughout the piece.
It also clearly established two PCs' characters to the other cast members—also in ways that made them all a little more sticky.