Act 2 scene 3:
Let me first remind you of scene 2: it's a very short scene and usually it is put either with scene1 or scene3, but many people prefer to put it as a small, short, separated scene because they want it to function as a COMIC RELIEVE or a BREAK for the audience because you will have a very serious, heated issue coming up very soon so they want you (the audience) to feel a sense of comic relieve and have a break between scenes. Another reason is basically, to give you a contrast between how relax and nice is the atmosphere that people are celebrating happily and with the scene that you will have.
Scene 3 is opened on Syprus . Othello, Desdemona, and everyone is out in the street, talking happily, Othello orders his lieutenant and soldiers to take a break from 5 to 11. He trust that his people will watch out. (when a problem finished like the Turkish fleet, people joined together and celebrate and something might happened) so, Othello has a very intelligent sense that people should have a short break to celebrate and then come back to guard in case something happened. That told you something about Othello himself that he realized that people should be watch all the time. He orders Cassio not to drink and emphasized that people should not be drinking because the situation is not really save and secure and the Turkish might come back and do something to us.
So, O. and D. go to celebrate their wedding. Now we have Iago whose taking advantage of Rodrigo, he is the card that Iago has been using again and again and again. Remember that he asked Rodrigo to pick up a fight with Cassio but he did not tell him why, all he needs to know is to pick up a fight with C.. that told you something about Iago that he does not trust anybody except himself.
Dose Iago planned everything from the beginning when he said "I will abused Othello's ears"? No, he is taking advantage of the chances and he is lucky because advantages and coincidences are serving him and...