Whether you are writing copy for an ad like Mr. Ogilvy, or standing on stage in front of a group of children at your local library telling ghost stories, you are always going to do better when you are better prepared. A story should not feel like it’s being read from a script. It shouldn’t feel like it has been staged. The only way you can pull off the kind of improvisation you will need for a great story is if you know the major points, the information that you are trying to convey, backwards and forwards.
Don’t try to memorize the details, instead get a feeling for the flow of the story. Speak it aloud and try to understand which parts carry the most weight, what you want to emphasis and what elements are the most important. Once you understand that, the rest can easily be embellished.