Consider the actions of Sebastian with Feste, with Sir Andrew, and finally with Oilivia in Act IV, scene i of Twelfth Night. "Are all the...

From the moment that Sebastian enters the play in Act III,
scene iii, the audience has been waiting for just the comic misunderstandings that
happen in Act IV, scene i.  Sebastian and Viola are twins, and so it must be (a comic
rule) that Sebastian will be mistaken for Viola in moments of extremity -- Andrew having
intended to duel Viola (Cesario) to the death over Olivia, and Olivia head-over-heels in
love with Viola (Cesario).  So, in effect, not only are Sebastian and Viola twins, but
Viola is pretending to be a guy named Cesario.  Wheee!  Comic complications
galore!

Sebastian's first confusion involves Feste (or
simply Clown) at the top of scene i.  Feste is insisting that Sebastian is Cesario
(really Viola) and that Olivia has sent him to fetch Cesario and bring "him" to her. 
Obviously, Feste confuses Sebastian for Viola.  Sebastian treats Feste as if he is a
crazy person who has randomly accosted him on the street, and after trying to simply
tell him to get lost, he gives him money.  Sebastian
says:

I
prithee, foolish Greek, depart from me.

There's money for
thee: if you tarry longer,

I shall give thee worse
payment.

Apparently,
Sebastian, unlike his sister, is not afraid of a good
fight.

Which leads us to the next hapless character to
encounter him, Andrew.  The last we saw Andrew he matched Viola in cowardice and was
unable to follow through on his duel against her.  Now the audience will get the violent
payoff that they have been waiting for, as Sebastian vents his agitation against Feste
on Andrew.  Andrew touches off Sebastian's violent side by striking him and saying, "Now
sir, have I met you again?  There's for you!"  While we can well imagine that the
cowardly Andrew merely taps Sebastian, Sebastian responds by beating Andrew to a pulp,
adding, "Are all the people mad?"

Sir Toby must pull
Sebastian off of Andrew, and when Toby claims that he will bring Sebastian to the
authorities, Sebastian draws his sword on the lot of them (by this time he faces Feste,
Andrew, Toby and Fabian).  Toby, not to be outfaced, draws in
response.

At this moment Olivia enters.  She throws herself
in front of Sebastian, whom she must defend out of love, believing him to be Viola
(Cesario).  She chides Toby and shoos everyone, save Sebastian,
offstage.

Here the tone of the scene does a complete 180
degree turn.  Olivia woos Sebastian with her unconcealed affection, and he suddenly
finds himself falling in love at first sight with Olivia, apparently forgetting, at the
drop of a hat, all the early strife of the scene.  He follows Olivia off in a lovesick
swoon.  He says:

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What relish is this?  How runs the
stream?

Or I am mad, or else this is a
dream.

Let fancy still my sense in Lethe
steep,

If it be thus to dream, still let me
sleep!

And so, as eager as he
was to escape the mad dream in the opening of the scene, here he is more than happy to
sleep and dream, if it means that Olivia will be with him.