Debugging is a separate mode. A script is either in debug mode or it isn’t. To turn on debug mode for a script:

• Choose Script > Enable Debugging. If the menu item is checked, debugging mode is on.

The script window appearance changes slightly, as shown in the illustration below:

Notice the indications in the above illustration that you’re in debug mode:

• The Step Over and Step Into buttons are enabled in the toolbar.
• The language popup has changed from AppleScript to AppleScript Debugger X.
• The gutter has widened and contains diamonds (to show where breakpoints can go).

To turn off debugging mode:

• Choose Script > Enable Debugging again, to uncheck it.

When you’re finished debugging, you will probably want to save your script not in debugging mode. A script left in debug mode (with its language set at AppleScript Debugger X) is not portable to machines that don’t have Script Debugger, and won’t run normally in other environments. (Apple’s Script Editor cannot even open such a script.) Having saved a script in debug mode, you would not want to distribute it to other users accidentally. (Also, a script in debug mode runs slower and uses more of AppleScript’s internal resources.)

There is, however, one good reason for deliberately saving a script in debug mode and running it elsewhere — so that you can debug externally.