DESDEMONA: Be thou assured, good Cassio,
I will do All my abilities in thy behalf.
EMILIA: Good madam, do: I warrant it grieves my husband,
As if the case were his.
DESDEMONA: O, that’s an honest fellow.

– William Shakespeare

Othello, Act 3, Scene 3. We see a striking example of dramatic irony here, as the scene opens with Desdemona’s assurance to Cassio that she will advocate for him to Othello. Both women believe that Iago is sincere in his dealings with Cassio. They are unaware, but the audience is, of Iago’s treachery, how he despises Cassio, and makes use of him to revenge himself on Othello.