In her post, she describes how her her characters evolve as she writes. Sometimes, it is in the interest of the story, but other times it's because she gets to know the characters better.
I've had the same thing happen--to the point where I end up changing the entire story because the character is leading me. And, sometimes, my protag evolves into an entire new character.
Does this happen to you?
One of the things I enjoy doing before and during writing a story, is to continually develop my characters. I fill out a character questionnaire or two and watch the world for interesting quirks, hobbies, habits, obsessions, mannerisms, etc. to apply to my characters to help develop them. When I'm in the middle of writing story, even when I'm not actually writing, my mind is always acutely aware of my story and its characters. So, when I notice something in a person in real life or on television, for example, I will directly realize that the trait I see belongs to my character. Sometimes these traits seem to come out of the blue.
I believe a well-developed character (both in the story and behind the scenes) will lead herself in a story. The reader should never be surprised (I am not referring to plot surprises here) in how the protag responds to or avoids things (and, yes, I know there are many exceptions).
Here are some interesting articles on character evolution and how to make your character interesting without becoming a two-dimensional cliched stereotype: