public function addCast($user_id, $video_id, $role = null) {
        if (!$this->getAccessToken()) {
            throw new Garp_Service_Vimeo_Exception('This method requires an authenticated user. '.
                'Please provide an access token.');
        }
        $params = array('user_id' => $user_id, 'video_id' => $video_id);
        if ($role) {
            $params['role'] = $role;
        }
        $response = $this->request('videos.addCast', $params);
        return $response;
    }