[ekster]

I agree that if it would make more sense for Frankenstein to just be happy having a wife and find comfort in her, abandoning his research and living happily ever after would result in a somewhat pointless 10 page book, but I think you can veer off and still have the same story to a certain degree.

Like in my original example, if I try to plan it out too much and go with "they need to break someone out, so character A steals the card and opens the door, character B stays outside as cover, on the way out, they get ambushed by 4 people, but saved by character C who comes in from the side," it would make sense in my story, and could work...
But when I start writing it, I find that maybe character B would be more true to himself if he doesn't stay to cover, but instead gets distracted by his urge to look around, and maybe character C wouldn't want to get involved physically, so he merely distracts the ambushing party and lets character B come back from the side to deal with them.

The story's main plot stays the same, the main character gets saved, but now that I actually got to the scene and put myself in my characters shoes, I simply found a more interesting alternative that also lets the characters be more true to themselves. This is a minor change, sometimes I'd end up with major changes, like a group ending up on the opposite side of the battle. But if their main object was to help a third party, the story itself still works, regardless on whose side of the battle they are on. It's just the getting there part that changed.