A huge portion of the cut footage from That Thing You Do! is a lot of deleted and extended moments with the film’s characters, especially our lead protagonist, Guy Patterson (Tom Everett Scott).
While the existing cut does a fine job of showing the friction Guy has with both his family and Tina (Charlize Theron), his girlfriend at the beginning of the film, the extended cut gives both plotlines more room to breathe.
So instead of just giving Tina a quick heel turn, we see her gradually falling for the man who wants to do nothing but crown her #15 molar, which works better than a random betrayal.
Not only do we see him getting hassled more by his family at the beginning of his career as a drummer, we also see Tina becoming more and more disinterested in Guy’s life in general, and her growing relationship with her dentist.