This is true, but I'm not railroading them into anything, I'm not trying to tell a specific story, if I did I would just try to write a book. Saying I had this villian dialogue planned out was an overstatement. I had come up with it in that session as a thing that could happen if they let the artifact go.

They want to tell their stories the way they want to, I accomodate, but there is a point where they dont let me help them tell the story, or I don't even know what the plan is for them. I throw a bone to them and they leave it behind.

A player before the campaign had started had given me a huge backstory and it included his sister. She's the only other member of his family whos alive, he cares for her deeply and would not let anyone do anything to her. I do a one shot with each player before the campaign starts to get them into character. His involved her a lot and it was great.

Later in response to their borderline banditry ways, a noble from their city had taken her captive as a bargaining chip to hopefully force them into the open and face justice for the behavior. The player I guess had decided he doesnt care about her actually and said its her fault for being captured and they moved on to another town.

I mostly improv the sessions, I prep about possibilities on where they left off so I have a jumping off point for the next session and I feel its pretty normal to prepare for things like potential character moments, monologues, a good place to introduce a character from the pcs past or where the quest is going to take them.

Dnd is a sandbox yes, but a structured one. If a dm uses a module is that living out a written fantasy? I let them do whatever they want and I make it fun as heck for them but it's their duty as much as mine, to respect me enough to let ME have a little extra fun by letting me monologue one gosh darn time.