In Gods on the Mountain, there's a character known as Celia, who works as a tavern girl in the desolate city of Dubris. Without giving too much away, she eventually becomes the love muse, and romantic interest, of another character. Originally, she had a much more minor role. In fact, I don't think I even named her. She was simply going to be a brief source of temptation for the protagonist to want to quit the mission and stay in the city, only to continue on out of a sense of duty.
Then I started reading the incomplete first draft to my wife. You must understand that my wife is a hopeless romantic, and believes if a character deserves true love, they should receive it. In this situation, she started wanting me to write that Celia and the protagonist ended up together. I fought with the idea in my head, trying to think if I could make it work, or if it would be worth the long-term character development. Eventually, I decided to go forward with it, and started writing with this in mind. By the end, Celia became a character who will more than likely pop up in any sequels.
Herein lies the importance of beta readers, and why they can be helpful in catching things you don't. Two of my beta readers complained that the relationship between Celia and the protagonist, after they first part ways, felt disjointed. One said that it felt like they were separated lovers, when their first contact was awkward and brief. I had to laugh at this, because Celia was, after all, written as a character who wasn't supposed to be relevant to the story, hence the disjointed feeling. I went back and rewrote the scene between her and the protagonist, making it longer, more fleshed out, and with a more developed bond between them. I sent the rewritten scenes to the beta readers, and they both agreed it was an improvement.
This entire episode was an example of how an author should be flexible with how he sees characters progressing in a story, and whether or not there's room for change. It's also a fine example of why you should reread and review your work if you make any serious changes to the story.