Deception is a primary theme of Act 2.
In 2.1, we find Hamlet playing his "madness" ruse on the unsuspecting Ophelia. She describes how he came to her chambers
...with his doublet all unbraced,
No hat at upon his head, his stockings fouled,
Ungarted, and down-gyved to his ankle,
Pale as his shirt, his knees knocking togetehr,
And with alook so piteous in purport
As if he had been loosed out of hell
To speak of horrors -- he comes before me (2.1.87-94)
In Act 2.2, the deception theme continues, this time with Claudius doing the dirty work; the "king" sends Rosencratz and Guildenstern, two former friends of the prince, to spy on him to see if his madness is real.
Act 2 also sets up the play "The Murder of Gonzago," a thinly veiled attempt to deceive Claudius into confession, for the play reflects the usurper's crimes.