Allocate the scene script a cast of personae for each of its entities.

        :param mapping: A dictionary of {Entity, Persona}
        :return: The SceneScript object.

def cast(self, mapping):
        """Allocate the scene script a cast of personae for each of its entities.

        :param mapping: A dictionary of {Entity, Persona}
        :return: The SceneScript object.

        """
        # See 'citation' method in
        # http://docutils.sourceforge.net/docutils/parsers/rst/states.py
        for c, p in mapping.items():
            self.doc.note_citation(c)
            self.doc.note_explicit_target(c, c)
            c.persona = p
            self.log.debug("{0} to be played by {1}".format(
                c["names"][0].capitalize(), p)
            )
        return self