when she's sitting on the bed naked

It wasn’t in the book or the original script. In an interview just before the film was finally released, Christina Ricci said during production that director Erik Skjoldbjaerg decided that a nude scene would show the character's complete emotional "nakedness.” Ricci agreed to do it but she said it was her first nude scene so she was terrified. Thankfully, she went on to perform more nude scenes in several of her later films.