Compare and Contrast how Iago sees Desdemona and how Cassio sees Desdemona.
Firstly, Iago's perception is informed by his misogyny. It is clear throughout the play that Iago has a generally negative attitude towards women. This becomes clear in his demeanour towards Emilia, his wife, and the conversation he has with her and Desdemona in Act 1, Scene 2.
Iago treats Emilia as if she is a servant. He asks favours of her which are demeaning. He, for example, entreats her to steal Desdemona's precious handkerchief, which Othello had given her as a gift. It is unfortunate that Emilia, eager to please him, obediently and without question, complies to his strange request.
In his interlude with Desdemona and Emilia in Act 2, scene 1, his disdain for women becomes apparent:
... you are pictures out of doors,
Bells in your parlors, wild-cats in your kitchens,
Saints m your injuries, devils being offended,
Players in your housewifery, and housewives' in your beds.
The remarks he makes of women here are quite bawdy. Without me going into too much detail, he is implying that women are the exact opposites of what they pretend to be whenever it suits them, suggesting that they are deceitful and misleading. Desdemona is quite disgusted by the slanderous nature of his comments.
When Desdemona asks him his opinion of her, he remarks that it is a difficult task to do so. He mentions in general, that if a woman is both attractive and wise, she would always, by using her intellect, find a way to exploit her beauty. The two continue in the same vein and Iago finally remarks that a beauty such as Desdemona is a spirit (or witch) as much as such a spirit can be.
Iago also admits in a soliloquy that he has feelings for Desdemona:
Now, I do love her too;
Not out of absolute lust, though peradventure
I stand accountant for as great a sin,
But partly led to diet my revenge,
For that I do suspect the lusty Moor
Hath leap'd into my seat;
He acknowledges that his 'love' for her is not born out of lust only, but that he has an ulterior motive in feeling as he does for her. He suspects that Othello had had an affair with Emilia and wants to take revenge for this. As he later states,
Till I am even'd with him, wife for wife,
Unlike Iago, Cassio has real affection for Desdemona. He is a handsome and charming character and one can perceive from his general behaviour that he is somewhat of a playboy, as illustrated by his relationship with Bianca, who falls in love with him. He forms a close relationship with Desdemona, who out of kindness after his dismissal by Othello, promises to intervene and speak to Othello on his behalf in an attempt to get him reappointed, which, we later learn, is a grievous error.
Iago believes that Cassio is in love with Desdemona:
That Cassio loves her, I do well believe it;
He plans to use that affection as a weapon against both Cassio and Othello, creating a pernicious and vindictive lie - that Desdemona and Cassio are having an affair.
Cassio's affection for Desdemona is clearly illustrated in his conversation with Iago about her. What Cassio unfortunately does not, and cannot realise at this point, is that Iago through his enticing remarks, is baiting him and setting him up for his vile plot.
CASSIO She's a most exquisite lady.
IAGO And, I'll warrant her, fun of game.
CASSIO Indeed, she's a most fresh and delicate creature.
IAGO What an eye she has! methinks it sounds a parley of
CASSIO An inviting eye; and yet methinks right modest.
IAGO And when she speaks, is it not an alarum to love?
CASSIO She is indeed perfection.