def do_execute(self):
        """
        The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        result = None
        teeoff = True
        cond = self.storagehandler.expand(str(self.resolve_option("condition")))
        if len(cond) > 0:
            teeoff = bool(eval(cond))
        if teeoff:
            self.first_active.input = self.input
            result = self._director.execute()
        if result is None:
            self._output.append(self.input)
        return result