What are some examples and quotes that support the theme of appearance vs. reality in Twelfth Night?

Well, the obvious place to start would be by examining the
character of Viola, and how she appears to be a man, but in reality is a woman. This of
course extends to other examples of appearance vs. reality, as her twin brother appears
to be to all intents and purposes Cesario, whilst in reality being somebody very
different. However, apart from these examples, one of the best quotes I think in the
whole novel that relates to this important theme comes in Act III scene 1, when Viola
discusses the role of Feste, and how he appears to be a fool, but in reality shows
himself to be incredibly wise. Note what she says:

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This fellow is wise enough to play the
fool,

And to do that well craves a kind of
wit.

He must observe their mood on whom he
jests,

The quality of persons, and the
time,

And, like the haggard, check at every
feather

That comes before his eye. This is a
practice

As full of labour as a wise man's
art;

For folly that he wisely shows is
fit,

But wise men, folly-fall'n, quite taint their
wit.

This quote relates to
the way that being a "fool" paradoxically requires great wisdom and great effort. Feste
is a character worthy of attention, because, if you analyse his role very closely in the
play, he is very definitely not what he appears to be.