The love triangle between these three characters is introduced by Orsino in the very first line of the entire play, where he confesses himself to be addicted to love, and this love is expressed towards Olivia. When Viola has disguised herself as a eunuch, she gains employment at the court of Orsino, and finds herself pressed into service as a reluctant messenger of Orsino's love for Olivia. The irony of this is, as Viola herself admits at the end of Act I scene 4, she herself is in love with Orsino:
I'll do my best
To woo your lady--[aside] yet a barful strife--
Whoe'er I woo, myself would be his wife.
The plot thickens when Viola, whilst wooing Olivia on her master's behalf in her male guise, so excites the passions of Olivia that Olivia falls in love with Caesario, not knowing that he is actually a she. Thus by the end of Act I the audience is presented with a potentially very comic situation: a lord in love with a lady who does not love him back who sends a henchman to plead his case to the lady who is actually a woman in disguise who loves her lord with the lady falling in love with this woman not knowing she is a woman.