The Services menu is available in almost all Mac OS X
applications. It provides a way that other applications can provide
extra functions, or perform some action on the selected text. Narrator
offers a service to other applications: choosing the > Services >
Narrator > New Document with Selection
command when you have some
text selected in another application (that supports the Services menu)
will launch Narrator (if it isn't already running) and create a new
document containing that text. You can then use the Casting Assistant
or add Characters yourself. This feature makes it very easy to use
Narrator to speak some text from wherever you come across it.
Note: Due to the way Mac OS X works, you need to install Narrator in the Applications folder, or a few other standard locations, for it to offer the service. Also, you will need to launch Narrator before the operating system will detect the service for the first time.