Desdemona is the perfect Renaissance wife in Acts II - V. Around her husband, she acts like the way women were taught to act. She is the virgin, the quiet woman, the good wife--very submissive, knows her place, defers to the male.
When Othello is not around, at the beginning of Act II, she trades barbs with Iago. She is witty, laughing at his misogynistic attitude toward women. Notice that Emilia does not partake in the exchange. Emilia too quietly plays the role of good wife.
But in Act IV, when they are in private, we begin to see differences. Emilia is the first to realize Othello is jealous. She admits that men are "but stomachs and we their food." This suggests that--even though she has been quiet up till now--she knows first hand gender differences in marriage and the double standards men employ against women. We get a sense that she does not love Iago.
Later, they discuss treating husbands the way they are treated:
I have heard it said so. O, these men, these men!EMILIA DESDEMONA EMILIA DESDEMONA EMILIA DESDEMONA EMILIA DESDEMONA
Dost thou in conscience think,--tell me, Emilia,--
That there be women do abuse their husbands
In such gross kind?
Herein lies the differences between the two: Emilia is much more worldly, pragmatic. She is not playing a role; she speaks the way a modern woman might--one who has been taught to question roles openly. Desdemona, on the other hand, is still the submissive woman who dare not treat her husband badly, even though he's about to kill her. She willingly martyrs herself to let her husband maintain his reputation.