Creative Evolution

When I first penned down my first novel, I had a clear road map of what I was about to write and how everything was going to proceed. However, I was up for a surprise! As I wrote down the narration, I found that my characters started to evolve with each passing time and the situation I had presented to them in the book. Not only that but the situation itself needed an amendment for them to grow and expand. Even as a writer, it baffled me to observe this peculiarity. For example, the narration was decided to be only restricted to a few characters in the book. But for me to write and convey a proper story, I did (and that too badly) needed a point of narration from other characters as well. And that eventually led the significance of these characters. By the end of the book, they marked themselves as one of the main characters. And their evolving personality defined and distinguished them, conclusively showing that their features are now trademarked to the personality itself. It is very mild surprise for me to discover this. Do other writers feel this way as well, that the plotline they initially planned never turns out to be the same?