Enable verbose boot:

For the repair volume


mount the repair partition:

mkdir /mnt/repair/
mount -t hfs /dev/disk0s2 /mnt/repair/
then edit /mnt/repair/com.apple.Boot.plist

under key: Kernel Flags
add -v before rp=:///OS.dmg

For the normal OSBoot Volume


edit /System/Library/CoreServices/com.apple.Boot.plist

under key: Kernel Flags
add -v