The main thing which attracts Olivia to "Cesario" is Cesario's forthrightness. As Cesario, Viola is plain when speaking with Olivia and is not too timid to critique her behavior regarding love. This comes easily to Viola not just because of her personality but because she is not attracted to Olivia and therefore is not interested in making a handsome impression.
Compared to Orsino, who seems more infatuated with a romanticized notion of Olivia than with the woman herself, Viola is honest. Olivia is attracted to Viola's plain manner, rather than Orsino's distant, ardent wooing, probably because Orsino never bothers to woo her in person; he only sends servants with sweet words and trinkets, which are nice but fundamentally impersonal.
One reason why Olivia falls in love with Viola disguised as Cesario is because Viola is extremely direct and forthright with her, which can be very attractive. Unlike Orsino, Viola is able to view Olivia through critical judgement, which is far more honest than mere...
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