One often hears authors describe how their characters take over and do things unexpected. This happens to me as well. More years ago than I care to reveal, when I first assembled a cast of characters and began writing about them, I had a female supporting character who kept on trying to shoulder her way into the limelight and make it her story. In my inexperienced mind she was only along for the ride, but she wouldn't be tamed, and in the end I gave up and switched mid-story into writing from her point of view only.
Back in those days I was writing by hand in A4 exercise books, and somewhere in a box are about 10 filled with scrawly handwriting. That story -- and the same female character -- was the nucleus for the novel I am desperately trying to finish now. It was one of my first lessons as a writer: listen to your characters and let them be heard.
I mention this because I am currently reading a 4-book fantasy series in which one of the major characters gets very little airtime in the first book. Speaking with a friend of the author, and someone who read an early draft of the novel, I discovered that this character was barely even present in the original draft. This smacks to me of a character who asserted his dominance as the story progressed, and who decided that he would be the hero of the story. This would have forced the author to go back and beef up his presence in the first novel, even though his role doesn't really take off until book 2. I love knowing this!
Part of the joy of writing is putting characters in situations and seeing how they react. This is particularly so in first drafts, when you're still figuring them out and allowing them to drive the story into places unpredicted. Not so much in rewrites, when you're trying to make sure they're consistent and believable. In this case, the thrill is knowing and understanding the characters so well that it's obvious what they would or would not do or say.