expect will only help if you can log in.
Check out /etc/shadow
that is where the password is actually stored normally, though an x is normally required in the passwd file.
This assumes you are using the shadow-utils package.
You could set a password, and it probably is advisable to do so, if the machine touches the net without a password root will be exposed especially if you run any software which allows a remote login, which means any data on the harddrive is potentially available as well as the machine being usable for the time you have the livedisk running.
root is already logged in, it is running init and bash.
expect will allow you automate things requiring command line input, but I think the first thing to do is to give root a password.
As the system will be root anyhow you can login in as whoever.
It sounds like your problem is to do with auto login really, link below should help:
http://gentoo-wiki.com/HOWTO_build_a...D_from_scratch