Most stories start with a character who wants something. That need, that desire, is what drives the story forward as the protagonist fumbles their way toward achieving that goal. I've read countless books, blog posts and magazine articles talking about the importance of making your character's need very clear from the beginning of the story so that readers know to cheer as the MC gets closer to his goal, or feel frustrated as obstacles keep him from the goal.
But can the goal change over the course of the story?
Say you have a character who is desperate to get back home. But the taxi breaks down on the way to the airport, she misses her flight and is stranded by the side of the road. While she waits for another taxi, a minivan crashes in front of her. She pulls the lone survivor from the burning vehicle, a child who is suddenly alone in the world.
How would this affect your character? Would it change her goal, at least for the short-term?
Say your character decides to help this child get back to his family on the other side of the country, even though it means delaying her own desire to return home. Now she has a huge obstacle. Not only is she going in the opposite direction of where she actually wants to go, she's also responsible for another human being.
The choices your characters make don't just affect their goals. They also reveal who they are as people. My imaginary character could have handed the child off to the police and caught the next flight out of town. But then what would that say about her as a person?
What about in your story? Do you have characters with changing goals? Is your protagonist sometimes his or her own antagonist as well?