So what's the secret of creating a great character? It helps if they're a little flawed, it makes me like them if they have problems to overcome. But the real secret isn't in how the character acts. It's how they react to the world around them. And frankly, some writers can't seem to write realistic reactions.
What Would J.K. Rowling Do?
Suppose that you were confronted by a vampire, just suppose. Pretend that you've just been given terrible news. Imagine exactly what you'd do if you were arrested.
Don't just write things down. This is not a viable way to craft a story. See yourself in this world. Envision how you would behave if all the plot twists were happening to you. Always take the time to do that. Just stop, think and imagine.
Because if you don't, your book isn't going to be very good. I can't cite any specific examples, because that would be mean, but I've read lots of books with unrealistic reactions. So I can point out some general examples of what that looks like: