Horatio is first brought into the plot because of his judiciousness and his extensive education: the guards call on him, as a scholar, to address the ghost that they have encountered. Horatio’s half willingly arrival: “A piece of him” creates an ominous atmosphere. We soon find out why, and the sense of premonition deepens when we learn of the appearance of an apparition, a contact with the beyond. Horatio's response on being told of the apparition shows his intelligence, just like Hamlet, he is an intelligent and perceptive man struggling to live among the deceptions, contradictions and stupidities of the only society he can know. A former fellow-student at Wittenberg, the centre of the German Enlightenment and the Protestant Reformation, it is understandable that he would doubt the existence of the ghost they have the presumption to assume that their senses and their machines can detect everything. This encounter introduces the theme of the conflict surrounding the Renaissance.
After seeing the ghost for the first time, Horatio makes an astute comment that sets the tone for the rest of the play: “This bodes some strange eruption to our state”. In this proclamation, he assumes the role of “he that knows”, an identity that constitutes an important part of his relationship with Hamlet and the basis of his importance in Hamlet. In keeping with his purpose in the play, Horatio makes it clear when Hamlet is making a choice that is ill-advised, and even illuminates the negative implications of the choice. When Hamlet meets the ghost for the first time and the ghost beckons Hamlet to follow it. Horatio immediately protests and asserts himself aggressively, affirming to Hamlet “You shall not go” and repeating it three times for emphasis and urgency. This behavior is emphasising the contrast in which Horatio has spoken to Hamlet so far. Upon Hamlet’s inquiry as to why, Horatio argues that the ghost may “tempt [Hamlet] toward the flood” which “might deprive [his] sovereignty of reason …And draw [him] into madness” Here, Horatio foreshadows Hamlet’s fate: he will lose his reason, toe the line of madness, and be ruled by “passion,” all of which will put “toys of desperation” (thoughts of suicide) into his mind. If Hamlet would have listened to Horatio, he may have been spared his fate.
The nature of the ghost is important in the play. Horatio questions its identity, an issue that also concerns Hamlet later. More crucial to the theme of the play, however, is whether the ghost holds any moral or spiritual authority. This is made clear subsequently. After the ghost's appearance, the guards ask Horatio the reason for the war preparations; his answer introduces the first of the four parallel actions in the play. The former king of Norway, Fortinbras, had been killed by Hamlet's father in single matched combat, and had also forfeited lands to the conqueror under the terms of a formal agreement. Now his son is the first character in the play who seeks to avenge a father's death.
The reference to Julius Caesar introduces the sense of self awareness in the play which is so crucial an element of Hamlet’s own examination of theatricality and its connection with ‘reality’. But it also serves dramatically to remind the audience that the good of people might sometimes require the assassination of an autocrat, as Caesar’s killers claimed. Horatio tells Bernardo and Marcellus that in ancient Rome just before Julius Caesar was assasinated "The graves stood tenantless and the sheeted dead/Did squeak and gibber in the Roman streets." The appearance of the ghost is a bad omen and Horatio warns that something evil will soon befall the nation, for young Fortinbras of Norway in violation of an earlier treaty is about to invade Denmark.