Set the DEBIAN_FRONTEND to noninteractive:
DEBIAN_FRONTEND=noninteractive apt-get ...
This is also applicable to dpkg --reconfigure, dpkg-configure, etc.
From man 7 debconf:
noninteractive
This is the anti-frontend. It never interacts with you at all,
and makes the default answers be used for all questions. It
might mail error messages to root, but that's it; otherwise it
is completely silent and unobtrusive, a perfect frontend for
automatic installs. If you are using this front-end, and require
non-default answers to questions, you will need to preseed the
debconf database; see the section below on Unattended Package
Installation for more details.
If you do set noninteractive, you should consider answering debconf questions by using debconf-set-selections.