Reading a play differs from reading a novel because in a novel the narrator says what is happening. In a play, the reader has to figure put what is going on, the interpreting the dialogues the characters are having throughout the play.
If "Twelfth Night" was a novel, it would be so much harder to understand the plot because Shakespearean language would make it very hard to interpret.
When reading a play, the imagery pictured in mind will determine the way the play and the characters are seen. In a novel the characters and the play are established in a certain way, determined by the director of the movie.