Customizing runlevels is an advanced system administration task. The following advice holds for most services.
To enable service service in runlevel R create the symbolic link /etc/rcR.d/Sxyservice with target ../init.d/service. The sequence number xy should be the sequence number that was assigned to the service when the package was installed.
It is convenient to use a runlevel editor such as sysv-rc-conf or ksysv for these purposes.
It is possible to delete the S symlink for a service in a particular runlevel directory instead of renaming it. This does not disable the service but leaves it in a “floating” state as far as the sysv-rc init system is concerned: on runlevel changes the service will be neither started nor stopped but will be left as it was, whether running or not running. Note, however, that a service left in such a floating state will be started if its package is upgraded whether or not it was running before the upgrade. This is a known shortcoming of the current Debian system. Note also that you should retain a service’s K symlinks in runlevels 0 and 6. If you delete all the symlinks for a service then on upgrade the service’s package will restore the symlinks to their factory default state.
It is not advisable to make any changes to symlinks in /etc/rcS.d/.