As the film was about to be completed , Katzenberg suggested to the filmmakers to redo the film 's ending in order to " go out with a big laugh " ; Instead of ending film with just a storybook closing over Shrek and Fiona as they ride off into the sunset , they decided to add a song " I 'm a Believer " covered by Smash Mouth and show all the fairytale creatures in the film .