"I thought I understood the story very well, because I've lived with it for so long. But movies change and take on a life of their own once they start to be made, and you have to keep your eye on the real ball, not the ball that's in your head"

This quote by Andrew McCarthy speaks to the concept that even if you think you comprehend something very well, it can still shock you. He is referring to the process of making a movie, which can be a long and complex procedure. Even if you have a clear vision of what the motion picture must be, it can take on a life of its own as it is being made. This suggests that you have to be flexible and available to changes, and not get too attached to the concept of what the movie need to be in your head. It is essential to stay focused on the genuine ball, which is the real motion picture that is being made, and not the one that exists in your head.