The description of her
death in the book is quite 'fun'.
Terry Pratchett apparently thought of it some time before for the
discworld. However he couldn't use it because he didn't want to kill off
Granny Weatherwax (think what she might have done to him if he had tried).
Safety Tip
If you are going to burn someone at the stake - first ensure that they have not filled their pockets with gunpowder and nails.