Auden's choice to stay with her dad, Robert, and step-mom, Heidi, for the summer is met with derision by her mother Victoria, a respected English professor. In the movie, Auden has already made this decision and is driving to Colby a few minutes after the opening credits. In the novel, it's a more drawn-out process, with Auden deciding to take Heidi up on her offer after spending a few lonely weeks at home.
Similarly, Auden doesn't plan to start working at Heidi's boutique, Clementine's, until she's in Colby and sees how an overwhelmed Heidi is trying to balance the financial side of the business with taking care of her newborn. On-screen, all these decisions have already been made.