The one thing we all hate is when we’re writing and we think that we’re in control, but we’re not. The characters speak and they do their own thing – turning your story upside down and changing your plot.
My best advice when this happens is to let them – Listen to your characters and don’t fight them. Them changing your story might be for the better and when you’re not fighting with your characters, the words flow better. Changing your story to what your characters want could very well be just what your story needs. Remember, you created these characters – they’re your babies and like any baby, they do grow up and they decide that they want to make decisions for themselves.
BUT if you don’t want to listen to them, not all hope is lost. You can still keep to your original plan and to do so, I would take a step back from your story for a day or two and start planning chapter by chapter for your book. Plan everything, the conversations, details, scenes, all to make sure that things stay the way you want. This is something that I don’t advise as your characters will push back (trust me, it’s happened to me) and it will make writing 10 times harder than it already is.