Although Viola agreed that she will obey, but deep down, she has already fallen in love with Orsino. She would rather be his wife than try to woo Olivia for him.
To woo your lady. Yet a barful strife!Whoe’er I woo, myself would be his wife. (1.4.45-46)
At Olivia's house, Maria brought Feste the Fool to cheer Olivia up. However, Olivia seems to dislike Feste's jokes. While they are arguing, Maria interrupts, saying someone is waiting to speak with Olivia.
I know his soul is in heaven, Fool. (1.5.67)
What is the point of mourning when your brother is already in heaven?
Madam, there is at the gate a young gentleman much desires to speak with you. (1.5.98-99)
Olivia thinks it must be Orsino's messenger again, so she refuses. Right after that, her steward Marvolio comes in and tells her the messenger will not leave before he sees Olivia.
Madam, yond young fellow swears he willspeak with you. (1.5.137) He’s fortified against anydenial. (1.5.143)
Tell him he shall not speak with me. (1.5.145)
Has been told so, and he says he’ll stand at your door like a sheriff’s post (1.5.146)