hamlet

In Shakespeare's Hamlet, the ghost of Old Hamlet serves as a tool to bring out the paranoia and vengeance in Hamlet, without letting the audience know if he is right or wrong to do so.
The first time the ghost of Old Hamlet appears, the audience just assumes that what Hamlet is seeing is true, because there is no evidence otherwise. Without any second-guessing, Hamlet begins to form his revenge against his Uncle, and the audience is almost forced to agree with him.
As the story goes on, the audience is able to gather more evidence about whether or not the ghost is a sign of Hamlet going insane, or a real apparition. When Hamlet tells his mother, Gertrude, to look at his father's ghost, she sees nothing. This is the incident that puts a question in the audience's mind regarding the reality of the ghost. As other happenings similar to this occur, the question gains more uncertainty.
Whether or not the ghost of Old Hamlet was all in Hamlet's head, or a real omen, it aided as a reminder to not believe in Hamlet's actions soley because he was the protagonist.