Locking your screen for privacy and security
The conundrum:
You might have something sensitive open on your computer (a private letter, etc.) and you'd like to be able to walk away for a few minutes without risking that someone could see your screen while you're gone. But logging out is annoying because it quits all your stuff and takes too long; and if it's set to automatically log you out or go to sleep/screensaver after X minutes, then INVARIABLY that happens while you're showing a movie or something, which is super-annoying. This is ONE way to compromise.
1. Go to the Apple menu -> System Preferences.
2. Click "Security and Privacy."
3. Click the "General" tab and check the box to "Require password after sleep or screen saver begins", and set the option to "immediately."
4. Click "Show All" (upper left of window) to get back to the main System Preferences window.
5. Click "Desktop & Screen Saver."
6. Click the "Screen Saver" tab.
7. Set the "Start after" option to "Never".
8. Click "Hot Corners" and select "Start Screen Saver" for the lower right corner.
See below for clarification, and follow the red text in 1-2-3 order:
Now, anytime you walk away and you want to lock your screen, just park your mouse cursor in the lowest right corner. The screen saver will come on immediately. When you come back with your coffee, wiggle the mouse or press a key, and you'll be prompted for your password to dismiss the screensaver. It's faster than logging all the way out, and it won't come on by itself and interrupt your presentation or movie (unless you accidentally slide your cursor into that corner!)