It's a linear adventure is it not? I mean, as long as the players want to follow the prepped plotline.
Even the most linear adventure, say Tomb of Horrors will provide different experiences to different groups. There will be a lot in common but how they dealt with or failed to deal with different traps and the different solutions that they applied to the Tomb will all be different. You'll have a lot in common, but you will have different transcripts of play. So even a linear adventure allows for Expression and has different walks through it on the "micro" level.
I am very curious...why do you think this? Is it that lack of prepped material will encourage a GM to force players to follow a prepped plotline?
A couple of reasons. First, a lack of prepped material means that the only thing on offering is what the GM imagines at the moment. And secondly, a lack of prepped material makes it much easier for a GM to force outcomes of scenes to be whatever the GM imagines is the most fun. Lack of prepped material tends to reduce the amount of "Sign Posts" - that is the number of things in the setting that players can interact with in order to find novel things in the setting. If you are improvising, the tendency is to provide zero to one sign posts. There are either no clues what to do next or just the one clue. The more you spend detailing the setting the more "sign posts" you have to lead to areas that you probably don't consider when improvising.
In both cases, players can head off into the fog without a clear or intended sign post and you have to improvise. In my recent game the players wanted to find out how the terrorist organization was getting information about caves since they knew from briefings that the organization used caves in which to hide and lose pursuers. While that was an "obvious" thing to do, I hadn't recognized this as a "Sign Post" when I prepared, so I had to improvise. The choices and density of information I could provide when improvising was certainly less than if I had prepared for this path ahead of time. And the number of alternative paths to what I was improvising was reduced compared to exploring a path which had more details.
The players don't get fewer choices to make when you detail the setting more. They get more. More importantly, they get more signals about what choices that are available. With few signals they tend to only go in obvious directions and that makes it really easy to consciously or unconsciously steer the players through a very linear series of bread crumb paths. The fact that those linear bread crumb paths were improvised on the fly doesn't make them less linear, nor does it mean the players had more choice just because you hadn't planned head of time for them. GMs without prep invariably thump down one sign post that then the players just follow along to the only location they can "see".
The less of the world that preexists the players interacting with it, the less agency the players actually have. If nothing comes into being until the players interact with it, then everything that is created is created in response to the GMs biases about the current path the players are on and the GM has vastly more options than one that has prepped if the prepared GM feels staying true to his preparation is a contract.
Think about chasing after a criminal. If I prep ahead of time how the criminal plans to make a getaway, then the criminal has limited choices and the players can make choices that thwart him. But if I don't prep ahead of time how the criminal plans to make the getaway, then I'm inventing resources for the NPC knowing now what choices the players are going to make. In effect, I'm always simply deciding by fiat whether I want the players to succeed now or not, since the criminal could have whatever plan would thwart the PC's particular choices. I can never know whether I would have, on behalf of the criminal, made the same choices while blind to the PC's actions or intentions or schemes.
