expect is not the proper tool, I think.
As already suggested, use either kdesu or gksu to prompt a user for root password before running root-only commands.
As an alternative, you can create a .desktop file for your script which pops up a terminal window when you launch it or, otherwise, you can use some GUI controls via zenity
(part of KDE, so if you're not using it don't even bother installing all of KDE just for it) or a Unity script
if you're using Unity as desktop environment.