This program demonstrates how to use a sprite to write on the Stage, using two different techniques: one uses a block I've created (a new feature in Scratch 2.0) and the other uses broadcasts to send the text to be written from one sprite to the letter writer sprite. For a detailed breakdown of how this works, see Chapters 7 and 10 of Scratch Programming in Easy Steps.
This program is an example from the book Scratch Programming in Easy Steps by Sean McManus.
