If you write, you might know what this feels like. If you read, you might be interested. Depsite working with a detailed outline, the characters still call the shots. I can't help it. It might be easier if they didn't.
Today, for example, I'm writing--minding my own business and suddenly I'm hearing from an important character, a confession. I wasn't going to make him a choir boy, but I didn't expect to hear about his membership in something shocking. He's not only a naughty aristocrat, I love naughty aristos, don't you! He's part of something that is really awful and answers a great many questions!
I can't spell it out other than to say, I'm amazed and I think my readers will be too!
No use guessing, all will be revealed!