sudo sed -i 's/\(GRUB_CMDLINE_LINUX_DEFAULT="\)quiet"/\1"/' /etc/default/grub
sudo sed -i 's/\(GRUB_CMDLINE_LINUX="[^"]*\)"/\1 nosplash noplymouth"/' /etc/default/grub
sudo update-grub
Caveat: This breaks anything requiring plymouth to interact with user interaction, likely debsel stuff.
also
init=/sbin/init -v