How does the following quote show a thematic approach to Hamlet?"We are oft to blame in this - 'Tis too much proved - that with devotion's visage...

This quote by Polonius is applicable to many of the
characters in the play -- first, and most notably, Claudius.  The quote means that
people who are guilty of something ACT in a way that they hope covers up the reality of
their bad behavior.  Claudius ACTS like a devoted father to Hamlet, but he is
responsible for Hamlet's father's death.  Claudius ACTS like he is greatly grieved by
his dear brother's death when he speaks to the court in Act 1, but again, he is the one
who killed his brother.  Claudius ACTS like a devoted husband to Gertrude, but he
doesn't love her enough to reveal the truth about the poisoned cup of wine, and he
allows her to drink it rather than draw attention to his evil plot against
Hamlet.

The theme of ACTING is prevelant throughout the
play.  The actors arrive to put on a play and ACT the parts of a king and queen, but it
is only a story.  Hamlet is struck by how "true" their presentation seems, even though
it didn't happen to them as people.

Hamlet is ACTING crazy
in order to distract and lower the guard of the people of the court, especially
Claudius.  He hopes that Claudius will reveal the truth of the murder of King
Hamlet.

Rosencrantz and Guildenstern ACT like friends to
Hamlet in order to try to help him over this difficult time, but they are actually only
doing this in order to curry the King's favor in the hopes of a
reward.

Polonius ACTS like he is concerned about Hamlet's
madness, but is really only concerned with proving his intelligence and worth in
discovering the cause of Hamlet's madness (even though we know he is
wrong).

The theme of acting and artifice in behavior is a
theme that runs through the entire play.  A careful consideration of all of the
characters would reveal many more examples.  The above list can you
started!