in Act I Scene 5, the countess falls in love with Viola, who is disguised as a man. She admires Viola/Cesario's speech, looks, and general attitude and style.
Methinks I feel this youth's perfections
With an invisible and subtle stealth
To creep in at mine eyes. Well, let it be.--
and then, at the very end of the act, she goes on to say:
I do I know not what: and fear to find
Mine eye too great a flatterer for my mind.
Fate, show thy force. Ourselves we do not
Which is her way of saying she thinks she is falling in love, but is unsure exactly.
In Act One, Scene Five, the countess (Olivia) met with a young messenger, Viola (disguised as Cesario), who wanted to delivered a love speech that was given by Orsino that was supposed to be sent to her. Viola was very eloquent as he lavishly praises Oliva's beauty and virtue to the top of the heavens and the bottom of the seas. Later on, he was deeply fascinated with the young messenger that she asked Cesario about his/her parentage and family tradition. Soon after Cesario left, she ordered Malvolio to sent after him and give him a ring as a token for appreciation. Later on, she had found out that she had fallen head over heels over Cesario, the love for him sinking very deep and very passionately.