Viola: I am the man [II.ii.25]
Viola is saying that she is the man Olivia loves, even though she is not really a man. This posses a problem for several characters in the play because it creates a love triangle. Orsino loves Olivia, who loves Viola, who loves Orsino. The confusion could be avoided if Viola's true identity were revealed but she continues to use her disguise. But while in her disguise she can never be with Orsino and Olivia will continue to persue Viola, so nothing can be resolved.

Malvolio: ...and yet to crush it a little, it would bow to me... [II.v.143-45]
Malvolio is talking about part of Maria's trick letter that does not seem to expressly be about him as the earlier parts were. But if he "crushes it a little" and bends the meaning he can work the letter to suit him and make it seem to be about him again. The diction of "crush" and "bow" further illustrates the idea that Malvolio feels he is above everyone and is of a higher class.

Viola: Then think you right. I am not what I am.
Olvia: I would that you were as I would have you be. [III.i.148=9]
Viola hints at her deception but Olivia does not pick up on it. Instead she flirts more with Viola and says she wishes Viola would act more the way Olivia wishes her to and return her love. But Viola cannot because she is not a man and is serving a man trying to win Olivia's love, a man she also happens to be in love with. Olivia, like all those lovestruck, wants the object of her desire to behave exactly how she wants them to, because otherwise her perfect notion of them is ruined.