(For later Android versions)
You may want to try ways to mirror the screen. (Method 2 involves mirroring over WiFi)
Or narrate the screen.
With a totally blank screen, you can:
# enable USB debugging (not shown here, but the prompt is in the upper 40% of the screen. Yes is on the right)
# verify the phone is recognized and has authorized your PC
adb devices
# unlock the screen on the phone (power button, swipe, enter password, etc.)
# send the backup command (adjust as needed and try quotes or escape characters)
adb backup -apk -shared -all file.ab
# Respond to the prompt on the phone
The prompt is full screen and the confirm button is in the bottom right, above the launcher tray (back, home, recent apps).

Once confirmed, there is nothing on the destination PC that shows it's working.
On the source phone, it will display each package as it copies them (seems that way).
The destination file will show as 0 KB (empty) until the process completes.
Use a reliable cable and leave the phone somewhere it can safely rest without moving or even touching it. It took around 30 minutes to backup a Pixel 2 (16 GB backup, about 64 GB of data on the device). Not everything is part of ADB backup.
If you ctrl-C the process on the dest PC, the dest file will fill over the next few seconds with the incomplete backup. The phone will also show more packages that it was iterating through to copy. You could maybe use this to verify the backup was working.
It's not clear what the collision policy is on sending a backup with the same name as an existing file.
adb restore
allows no selection, all-or-nothing only). Otherwise: Save I'd say. After all, that's what Helium Backup does to sync apps/data between devices. – Izzy Feb 22 '14 at 21:28