He already believed that Claudius murdered his dad. The apparition tells Hamlet almost exactly what he wants to hear. This is what, at the end of Act Two, makes Hamlet suspicious that it might be a devil trying to trick him into committing a murder. It might make us, the audience, suspicious too. Back to the top Stagecraft 1.
Tension is created through the nervousness of the guards and their ignorance about what is going on.
It is increased through the enigmatic appearances of the silent ghost and the revelations that the country may soon be invaded. It is surprising that the Ghost should appear twice. Its first appearance is expected from the moment it is mentioned, but no-one would guess that it would appear again only a few minutes after leaving the stage. This scene misleads us as to what kind of play 'Hamlet' is. Though both these elements have their part in 'Hamlet', it is hardly its main thrust.