Allow your characters too much freedom and they will sprint away from the story. You want them to be adventurous, but within the confines of the page. Once they get out...well, things just get messy.
It's really in the early chapters that a writer must get a firm grip on the behavior of the main characters. Like the director of a play, you must give cues. Tell them where to enter. Set up the scene and let them know what they are expected to accomplish. Then sit back and let them act. They'll test you at first. Lizzie is currently trying to get the upper hand on me. But I think maybe today we'll come to some sort of understanding.