So what? Following a script is not exercising agency. I simply do not agree with your definition, I think it is useless. Whose mouth speaks the words is immaterial. Who originated the ideas is what matters. By your definition reading a written by another is exercising agency. That is nonsensical.
if you choose to plagiarise another creative then you have also seriously limited your own creative agency.
I am not talking about "creative agency". As I posted not very far upthread,
I think it is pretty fundamental, if we are going to actually understand how RPGing works, to separate who authors stuff in the abstract from who makes stuff part of the shared fiction as part of the process of play.
When a player rolls up a Dwarf PC and calls that PC Gimli, they are of course borrowing from JRRT. When a player rolls up a paladin PC and decides that their paladin is on a quest to regain the sword that will unify the kingdom, they are of course borrowing from Arthurian legend and all the many authors (including JRRT) who have themselves borrowed from that material.
This shows us the players in question aren't fully original. But it doesn't have any bearing on the degree of agency they exercise as players of the RPG in question.
Likewise for the GM.
I am talking about
who establishes that <such-and-such a thing> or <such-and-such a series of events> is part of the shared fiction. The module author does not make the stuff they wrote part of any shared fiction. All they have done is write a book. It is the participants at the table who, among themselves, establish the shared fiction. And in RPGing which allocates roles among those participants in the traditional way (GM and players) and in which a module is being used in the traditional way (ie as a source of backstory that the GM draws upon), it is the GM who is doing the bulk of that establishing, using the module as a source.
In my last Torchbearer session, I used material from the Tower of the Stars adventure in the Cartographer's Companion. That scenario was written by Radek Drozdalski. But he was not exercising agency over my RPG session. A year or two ago I ran a session of White Plume Mountain using an AD&D variant. Lawrence Schick wrote the module, but he was not exercising agency over my RPG session.
You're fixated on the timing for some reason. In such a situation the GM would have had exercised their agency prior, when they created those notes. Why it matters when they did that? What if they improvise the exact same stuff during the play?
I am not focusing on timing of authorship as such. I am focusing on
who makes stuff part of the shared fiction. If a GM authors notes, reads from a module, or makes stuff up - if they are principally responsible for establishing the content of the shared fiction, then that's that.
Timing of authorship is relevant only when it feeds into processes of play. Eg in classic dungeon crawl play, authoring
in advance so that the puzzle and challenges are fixed in place at the time of actual play is crucial. Gygax's advice to players in his PHB, for instance, makes zero sense if the GM is not bound by the map and key that have already been authored.
And in a very different paradigm of play - "story now" - then there are certain things that cannot be authored in advance, because the process of play demands that they be established in response to inputs and constraints that are themselves generated only during the course of play.
I have also to say that wholesale copying characters from media and putting them into your game that is presumably not set in the world of that media is something I find very strange. I used to do it as kid, but it certainly is not something I would today consider a good practice. YMMV.
I don't know what you mean by "good practice". I don't feel any shame in the fact that JRRT provides a wealth of compelling tropes and characters that goes beyond what I am capable of. I also don't think that the Greyhawk setting that I default to is so pure that it will be corrupted in some fashion by the Tolkien-esque allusions of me and some of my players.
When you play D&D do you expunge all the Hobbits, Ents and Balrogs?