So in your way, you're using the players input to make the situation concrete and in my way it is concrete before they ever get there.
I do chafe a bit at good GM's being flexible in the way you suggest. I honestly much prefer my way of doing things and would like to see it more widely adopted.
Ehhh....not quite...
My example does not let the players
decide anything.
My example allows
the GM to update and change their own ideas on the fly.
I think you are missing the "purpose" of being flexible.
By allowing the fiction at hand, the actions of everyone in the scene (and the ideas batted around the table by the players out of character) - to inform the "best choice for best story for everyone"... you have to allow change to whatever you prepped. That change is always up to the GM,
always.
The purpose of this flexibility (
especially when it comes from player comments) = is that there are times when players will dig much deeper and much harder into plots they feel followed their choices in game.
Remember... The purpose of a GM is to set the stage and run the show = but the
player's purpose is to make the choices that drive the plot.
So when a plot comes up (the bandit leader and the local lord), listening to the things the players talk about and tweaking NPCs and their backstory to be a refined and informed version now that things are in motion = will make any game better in all respects.
Maybe a PC wants a love interest = alter the bandit leader to be influenced best by a lover, that fits plot
and PC desires.
Maybe the PC assumes and is treating all bandits like terrible monsters = change the bandit leader to be the psycho driver of these poor, abused, and terrorized bandits who do his bidding. This gives the PCs a reason to not kill every bandit and to try and use an alternate method to take out the bandit leader.
Maybe the PCs could care less about a bandit with a heart of gold = let the lord see the carnage in his realm and demand the PCs kill every bandit they see. If they do, let their villainy follow them into later adventures. break their hearts when they find a young son's drawings of his mother and his father.
...
What I describe in the above examples are not all forced onto the GM. They are just a GM considering their players and what story will hit them hardest, make them really cherish the interactions even more.
By being inflexible, and demanding the bandit lord is only out for one means of action, and the bandits only ever behave in one fixed way = you are significantly reducing the chances of making the game the best it can be (or worse).
....
Final note: Super Secret Squirrel time = the players don't, and almost never will = know any of this. You can change your idea 10 times before it resolves with them = and they will never know!
Again, the GM made that sht up in the first place, so
it does not matter in any way whatsoever if that idea changes mid-flight.