The mood (and consequently Othello's mood) of the play changes at this very important part of the work. At the beginning, Othello is feeling sorry for himself that he has married and that he has gotten himself "cuckolded." The mood changes quickly to one of black vengeance, but back again to doubt. After Desdemona comes to tell Othello of dinner, Othello continues to stew and ruminate on the poisoned information Iago has planted in his ear. Othello tells Iago he has "set me on the rack" (3.3.385). What is exceptional here is that Othello's weakness (unbridled passion) is carrying him away into a fury that benefits Iago's machinations. Othello even nearly loses himself in "Othello's occupation's gone" (3.3.409). Othello hasn't confronted Desdemona, or seen any proof, but he's still going completely ballistic convincing himself of her infidelity. He is, quite literally, "eaten up with passion" (3.3.446).
Plot wise, this is the part of the play where Othello is poisoned with Iago's lies and deceits and where Othello asks for some "ocular proof". This is where Iago's psychological fish-hooks and barbwire entangle the Moor's passionate mind.