When logged in to my user account, the screensaver selected in System Preferences is used. However, when on the login screen before logging in or using fast user switching a different screensaver is used. How do I set the screensaver used on the login screen?
The screensaver for the login screen is the one set for the root (i.e. system) user account. To launch System Preferences as root, use sudo; a Terminal window run:
sudo /Applications/System\ Preferences.app/Contents/MacOS/System\ Preferences
Then you can change the screensaver settings as you normally would. Note that in order to use sudo, you needs to be an admin user.