Drupal 7 add ajax listener directly to node form
This interceptor_dvd_listener_callback() should return value, in documentation:


  After form processing is complete, ajax_form_callback() calls the
  function named by #ajax['callback'], which returns the form element
  that has been updated and needs to be returned to the browser, or
  alternatively, an array of custom Ajax commands.


Why don't you use this:
https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#states

It is drupal 7 standard feature to hide or show some fields when some elements has some states.