Sanity is in the mind of the beholder. Whether Hamlet truly goes insane in Hamlet, or if he is just acting mad to achieve his goal in avenging his father's death depends on each reader's interpretation of the play. There are often inconsistencies in Hamlet's behavior, sometimes it seems his clarity of thought is amazing, while other times it seems understandable to believe he is really insane. However, to continue and reverse his personalities to the point of convincing the audience that maybe he is actually insane, is undoubtedly the talent of only a very sensible person.
Hamlet meets with his father's ghost and is told that Claudius murdered him, and that he is to take revenge for this evil act. Had the guards and Horatio not also witnessed the ghost then we may really have reason to believe Hamlet is insane. For as Horatio states, "Before my God, I might not this believe, without the sensible and true avouch of mine own eyes."(I.i.56-58) After this encounter with the ghost Hamlet speaks joyfully with his friends thereby acting as if the news is positive rather than tragic. Here we see Hamlet's ability to manipulate his behavior to attain the effect he would like. Obviously Hamlet is not cheerful at this point; nevertheless, if the guards would know the essence of the information he was given they would suspect him later.
One must keep in mind that immediately after meeting the ghost of his father he informs the audience of his plans to fake madness. He decides this shortly after Horatio's warning:
What if it tempts you toward the flood,
my lord, Or to the dreadful summit of
the cliff That beetles o'er his base into
the sea, And there assume some other
horrible form which might deprive your
sovereignty of reason, and draw you into
madness? Think of it. (I.iv.69-74)
This may be what gave him the idea to pretend to be insane.
After meeting with the ghost, Haml...