How do you boot verbosely when booting off an alternate device? I've got a problem where boot is failing when booting off a USB stick.

So, I hold option down to choose the USB stick, but if I immediately hold down command-V after, it still boots with plain grey screen.

What do I need to do to see the boot process when booting off an option chosen device?
Before rebooting, open Terminal and enter sudo nvram boot-args="-v". This will mean every future boot will be verbose and no keyboard shortcuts are needed. To disable, simply reverse by setting sudo nvram boot-args="".

Further, from https://support.apple.com/en-us/HT201573:


If you're using FileVault, release the keys when you see the login window. Then log in to continue startup in single-user or verbose mode.
If you're using a firmware password, you must turn off the password before you can start up in single-user or verbose mode.