What's a more graceful way to hide private information during a live demo Sometimes I give live demos from my Macbook, whereby I plug in an HDMI cable and show my screen on a projector.

During such demos, I sometimes might need to look up a login password which I keep stored in an encrypted file.

What's a better way besides unplugging the monitor to privately copy this password into the pasteboard without flashing my entire password file on the big screen?
You could hit Command + F1 which toggles display mirroring. Then use the laptop screen  to do the copy and paste. That way the users still see your main desktop, while you work on your laptop screen.