The Casting Assistant helps you to add Characters and mark up the text. You can get it to create the Characters for you, or you can create them and let it simply mark up the text for you. Here's how to use it:
Create a document, open a text file, paste text in, or use the Services menu from a web browser or elsewhere, that is formatted using a simple format of "<name>: <speech>", or similar. The requirement is that each paragraph start with the Character's name followed by some punctuation separating it from the body of the text (the Character's speech). Any text enclosed between brackets will be ignored, making Narrator compatible with the format used by instant messenger chats, so you can have it read back through your conversation.
If you want to set up the Characters yourself, add them as normal, using the same names as in the document. The Casting Assistant will use those Characters, if present, or add new ones as necessary.
You should then either select the portion to process, or have no selection (i.e. a flashing insertion point) to mark up all of the text.
Choose the "Director > Casting Assistant" menu command, and a sheet will slide out confirming the action.
It will then mark up the text, using the existing Characters, or adding new ones with random marker colors and Actors if a matching Character doesn't already exist. You can later change the marker colors and Actors to suit.