That's when ldebdiff does its thing: it runs diff against the original files of a given package and the ones installed on a system.
For example:
$ ldebdiff acpi-fakekey --- unpacked/etc/init.d/acpi-fakekey 2012-04-05 05:14:21.000000000 -0500 +++ /etc/init.d/acpi-fakekey 2012-04-12 12:10:55.000000000 -0500 @@ -4,8 +4,8 @@ ### BEGIN INIT INFO # Provides: acpi-fakekey -# Required-Start: $local_fs $remote_fs -# Required-Stop: $local_fs $remote_fs +# Required-Start: $remote_fs +# Required-Stop: $remote_fs # Default-Start: 2 3 4 5 # Default-Stop: # Short-Description: Start acpi_fakekey daemon($remote_fs implies $local_fs)