In Act IV Scenes 1 and 3, Olivia mistakes Sebastian the twin brother of Viola to be Cesario with whom she has fallen in love with. In Sc.1 she authoritatively breaks up the quarrel between Sebastian and Andrew Aguecheek and Sir Toby Belch and scolds Sir Toby in the following words:
However, she draws Sebastian aside and affectionately controls him saying, "nay, come, I prithee; would thou'ldst be ruled by me!" Naturally, Sebastian is pleasantly surprised but willingly surrenders himself to be ruled over by her.
In Act IV Sc.3, Sebastian in his soliloquy reveals that he is too amazed at her strange conduct and wonders whether all what is taking place to him is real and finds it difficult to believe his eyes and is convinced that either he or Olivia is mad,
"that I am mad
Or else the lady's mad."
but he reasons that she can't be mad because,
From Olivia's words and actions we learn just as Sebastian does that Olivia is the mistress of her household and is an authoritative person who is accustomed to giving orders and having those orders obeyed. For she soon enters the scene with a priest and leads Sebastian to her chapel to get married to him :
"Blame not this haste of mine. If you mean well,
Now go with me and with this holy man
Into the chantry by: there, before him,
And underneath that consecrated roof,
Plight me the full assurance of your faith;
That my most jealous and too doubtful soul
May live at peace."
The last two lines reveal that Olivia is jealous and possesive just like any other normal young woman who is in love.