As Hamlet, Horatio, and Marcellus wait until just after midnight to see the ghost, Hamlet and Horatio converse. When Horatio hears a flourish of trumpets and cannons going off, he asks Hamlet what they signal. Hamlet replies that the king is awake this night in order to have a drinking party in which he drinks wine from Rhineland and dances a German dance, the kettle-drum and trumpets announce his having drunk all his wine down. Then, Horatio asks if this action is a custom, Hamlet replies that it is, but it is a custom that receives more respect in the breaking of it than in the following, for the Danes are labeled as drunkards. Thus, their worthy attributes are mitigated. Hamlet bemoans the fact that many Danes who have sterling qualities are labeled simply as corrupt for having the one fault of loving to drink so much:
...The dram of evil
Doth all the noble substance of a doubt
To his own scandal. (1.4.58-60)
The one fault often causes doubt about all the virtues that men have, to their own disgrace. With this line and conversation with Horatio, Hamlet presages his own perceptions of the corrupt court of Denmark of which he is prince, one in which he finds no redeeming qualities.
After this conversation, Hamlet and the others see the ghost of his father, King Hamlet. When the ghost beckons Hamlet, he would follow, but his good friend Horatio cautions him, fearing that the ghost will lead him to a cliff and transfigure into something which will draw Hamlet "into madness" and make him lose his life. But, Hamlet with fortitude insists, saying that his fate draws him on and he must follow the ghost. Nevertheless, Horatio and Marcellus follow him, hoping to be able to aid him if necessary.
First of all, we learn from Hamlet that Claudius enjoys drinking and partying, that it is his custom to drink heavily, and that Hamlet very much disapproves:
it is a custom
More honored in the breach than the observance.
Secondly, Hamlet explains his views about the corruption of men--"The vicious mole of nature." He explains that men have a small defect that can convert an otherwise noble man into an evil one.
Thirdly, we know that Hamlet and Horatio both see the ghost, so that the ghost is not a figment of Hamlet's imagination, that he is not mad.
Fourthly, we know that Horatio cautions Hamlet against following the ghost--that the ghost may tempt Hamlet to madness. Hamlet, in turn, is determined to follow that ghost. We can surmise that Horatio is a loyal friend to Hamlet, that Hamlet is not afraid to speak to the ghost, that Hamlet is sane when the play begins, and that Hamlet is also quite moral.