Shakespeare's Hamlet takes a serious look at the theme of "appearance vs reality." This theme is a common occurrence in life; in this story, Shakespeare demonstrates his belief that such behavior can have deadly results.
"Appearance vs reality" is seen when the King and Gertrude encourage Hamlet to pull himself together regarding Old Hamlet's death. Gertrude asks why his mourning seems to be so difficult. He responds that it is not something that "seems:" what she sees is real, not just for show. Hamlet defies the idea that his actions are anything but as they appear.
...Thou know'st 'tis common; all that lives must die,
Passing through nature to eternity.
Ay, madam, it is common.
If it be,
Why seems it so particular with thee?
Seems, madam! nay it is; I know not 'seems.'
'Tis not alone my inky cloak, good mother,
Nor customary suits of solemn black,
Nor windy suspiration of forced breath,
No, nor the fruitful river in the eye,
Nor the dejected 'havior of the visage,
Together with all forms, moods, shapes of grief,
That can denote me truly: these indeed seem,
For they are actions that a man might play:
But I have that within which passeth show;
These but the trappings and the suits of woe. (I.ii.76-89)
Another example is when the Ghost appears to Hamlet: it could be his father's spirit or it could be a dark spirit bent upon destroying Hamlet's soul. To kill a king, which is what the Ghost asks of him, is a sin against God; the Elizabethans believed that monarchs were chosen by God. If Hamlet does not have just cause in killing Claudius, he will be committing a mortal sin. He needs proof that the spirit is "honest."
Once Hamlet gets these details from the Ghost, he tries to collect proof that Claudius did kill Old Hamlet. Hamlet decides that he will pretend to be insane when it suits his purpose. In Act One, scene five, Hamlet states that he will present an "antic disposition," and that seeing such, Horatio must not act as if this craziness is anything but real.
But come— Here, as before, never, so help you mercy,
How strange or odd some'er I bear myself—
As I perchance hereafter shall think meet
To put an antic disposition on— (189-192)
We see this theme again when Ophelia speaks to Hamlet while being spied on by the King and/or Polonius. The men are trying to ascertain the cause of Hamlet's mental collapse. Hamlet is aware that things are not as they appear—that Ophelia is collecting information for the others. (He blames her for this, but what choice does she have?)
Later, Claudius tries to make it appear that he cares for Hamlet as a son. However, it is a lie. After the "play-within-a-play," Hamlet has shown how dangerous he can be. The King tries to have him executed in England. Claudius lets Laertes believe that Hamlet is responsible for all of Laertes' heartache so he will kill Hamlet; the King says he cannot do so because Hamlet is loved by the people, and Gertrude dotes upon her son. Claudius says he is a friend of Polonius and Laertes, but this is also a lie. Later, Claudius acts like he loves Gertrude, but lets her drink the poison intended for Hamlet.
The false "fronts" the characters adopt lead to the death of all of the major characters. Ophelia is the only death that is not based on taking part in palace intrigue: murder, incest, and subterfuge. (She is an innocent.) The false appearances lead to the doom of the royal family and its household.