The tone in the opening scene of the play is mysterious. It begins with one guard challenging the approaching person so that immediately the audience gets set up for a story with lots of drama and action. In l. 25, Horatio asks if the "thing" has appeared tonight. The audience does not know yet what the "thing" is to which he refers, and the audience doesn't find out what Horatio, Marcellus, and Bernardo are talking about until the ghost itself appears. Shakespeare creates the tone of mystery by not naming the "thing", but just letting it appear. Add to that the fact that the ghost appears once again in the first scene and it never speaks. Also, the question of Denmark's war preparations creates this air of mystery. The audience knows nothing yet about why the ghost appears, about what its importance is, and they know very little about who the ghost is the spirit of or the political situation in Denmark.