But even when you’re writing a book, by the time you turn in a manuscript it still isn’t technically done. An editor might make suggestions, for instance. There’s an entire invisible element to writing that readers and/or viewers never consider, that it’s not merely a matter of writing the thing. But even in the initial writing, so many things can and should change. You don’t turn away inspiration, especially if it makes the story better. A writer worth their salt is always tuned into their fount of inspiration. You can’t turn it off. I don’t trust any writer who can.
I don’t know, it occurs to me, if you were thinking of comic books specifically. If it’s a short arc (enough for a single collection, say), I would hope it would be a story the writer has plotted out ahead of time. But the details can change. I get a little caught up in this sort of thing, being a writer myself. The destination is essential; it’s like a roadmap, or GPS. But you can play around with how you get there. Changing those details could very well make the difference in your perception of the destination. For instance: omit “How did you know that name?” and a certain neck-snapping, and how does perception change?