The difference between a horizontal and vertical storyline
A horizontal storyline is the story in itself: the plot with the characters and all the adventures they have.
The vertical storyline of a story, book or movie is the atmosphere and the emotion and feelings that come along with it. What we expect from you is not only to explain and show to us the plot of your story and tell us all about the characters, i.e. the horizontal storyline, but also to let us ‘feel’ the story, i.e. the vertical storyline. You can achieve that by, for instance, acting out a scene from the story of play or sing one of the songs from the story.