After many tellings of the same tale, the words you use may become familiar. There eventually may be a scripted text built out of your own vocabulary for the telling. Yet, as new flourishes and details are added, the story can continue to change and grow. Audience reaction can help shape the telling, too. Storytelling is a community event.
Listeners can offer constructive direction by their immediate response to the tale.
The teller must be open to how the tale is being received, so that its presentation can evolve. Eventually a finely crafted story may emerge using this development technique of improvisation. Yet, at any point in the development process, listeners can enjoy the tale. It is in the process of telling and retelling that the story becomes a work of art. The storyteller's words are like a painter's colors.