Whether you write to an outline, or churn it out by the seat of your pants, all of us are likely to come to a moment in the creation of a story where we are struck by a realization: if character X only did this instead of that, it would be so much better for the story.
How much trouble this causes you chiefly depends on how far along you are in the story at that point. It may cause you to revise your possibly painfully detailed outline, or it may force you to reconsider exactly where the story should be headed, if you're not confined to an outline, thus revising some of the highlights to come.
As a pantser instead of a plotter, I find this mostly happens to me when I brainstorm “what next?” for the moving boundary of the current words, but it can strike for older character actions, too, further back in the body of work.
I don't know about you, but my typical reaction when this happens is first elation (whoopee — an improvement!) followed by depression (look at all the changes I'm going to have to make to what's already been done and what is to come). Generally the former outweighs the latter, but I do have to get past the moment of deep despair at the work involved.
It could be worse. We could be George R. R. Martin.
If you've been living under a rock for the last decade or so, I'll just do a brief summary. GRRM has published 5 volumes so far of a series entitled A Song of Ice and Fire. The 1st book was released in 2003 and the 5th volume was released in 2011. There are more volumes to come (2? More?). Total pages published so far: 5100 — that's 1000 per book, on average. This is an immense professional epic fantasy spanning an entire world and dozens of major interacting characters with extensive multi-generational back histories and languages.
HBO came along and seized upon these books for a multi-season series, retitling it all as Game of Thrones. (Maybe you've heard of it…) The first season was broadcast in 2011, and the 8th and final season is happening now, in 2019.
How can I say “final”, when GRRM has not finished writing the series?
Well… GRRM has planned out and partially written some significant amount of the remaining work, and he has worked closely with the HBO screenwriters to share with them at least some of what will happen in the remaining books of the series.
As you might imagine, his fans are clamoring for the actual books. They've been making their voices heard since the 5th one came out in 2011, so for 8 years there has been an entire mini-industry of writing jokes on the subject of waiting for GRRM, or snark about the money he must have earned for the film rights.
But let's think about this as writers for a moment…
I have put off re-reading A Song of Ice & Fire until the final film episode completes (or almost) so as not to disturb my pleasure and suspense regarding the story conclusion, but I spent the last week doing the re-read, and am just a few hundred pages from the end (a day or so).
And in the process, it has hit me just how impossibly difficult it would be for anyone to now complete the writing of the books. Perhaps if GRRM had been able to go heads down on the last book or two, for releases in (say) 2014 and 2017 — staying ahead of the film — it would have been possible. But now?
Books vs film versions
Unlike a 2-hour movie version of a fantasy novel, the HBO Game of Thrones is 73 episodes of one or more hours each. This is a great way to do justice to epic fantasy, but film and books are different media with different strengths.
Film makes all characters recognizable, whether or not you can remember their written names. It imbues all of them, heroes or villains (or dragons), with a certain amount of charisma, since their every action can be shown rather than described. It lets you leave walk-on characters unnamed in large scenes, so that you aren't overwhelmed as a reader is with a superfluity of pointless names.
Books can do a better job of transitions between scenes, moving people along maps, explaining situations. They can “tell” where appropriate, where films mostly have to “show”.
But 73 hours is not and will never be 5000 pages (plus the missing 1-2 volumes). If the final page count were to be 7000, that's 100 pages per 1 hour of screen time. Sometimes that could work, but probably not consistently for the entire epic.
So films simplify. They do that because they have firmer constraints than the written word. They gain some ground by not having to describe as much, but not enough. Typically, they look for ways to simplify the plot complexities. Sub-plots can be pruned, important scenes repositioned, and so forth. More notoriously, they can make real substantive changes: killing characters who survive in the books, or keeping them alive when otherwise dead (e.g., dead in the books: Sandor & Gregor Clegane; alive: Catelyn Stark).
Every writer who sells a story to the film industry will face these issues, but they've already written their book(s), where the real story has been told.
The problem of influence
Now, GRRM is free to ignore the changes imposed upon his story by Game of Thrones. He is under no obligation to resurrect his dead characters, etc. But he can't unhear the discussions he's had with HBO, even if he doesn't watch the broadcasts. There has to be a real sense for him that the story has now been told, even if it's no longer his own story.
How easy can it be to go back to the immense work of bringing his plotlines and characters to their original conclusions, or to whatever changed conclusions have evolved in his own imagination? That's got to take the heart out of you, I would think.
I don't think I could do it, myself.