Madness is a consistent theme in William Shakespeare's Hamlet, and most of it centers around the protagonist, Hamlet. It is true that the appearance of the Ghost makes Hamlet and others question reality (and their own sanity for believing the Ghost exists), and Ophelia suffers from some form of madness after her father's murder and Hamlet's apparent betrayal. The majority of the madness, however, concerns Hamlet.
After he meets with his father's ghost, Hamlet resolves to avenge King Hamlet's murder by killing Claudius. Though he does not reveal a specific plan, he asks his closest friends a favor:
Here, as before, never, so help you mercy,
How strange or odd soe'er I bear myself,
As I perchance hereafter shall think meet
To put an antic disposition on,
That you, at such times seeing me, never shall,
With arms encumber'd thus, or this headshake,
Or by pronouncing of some doubtful phrase,
As 'Well, well, we know,' or 'We could, an if we would,'
Or 'If we list to speak,' or 'There be, an if they might,'
(The entire section contains 613 words.)