Iago is giving the plan to Roderigo about stabbing Cassio. And Roderigo says he might mess up so stay close.
Here, stand behind this bulk, straight will he come. Wear thy good rapier bare, and put it home. Quick, quick! Fear nothing. I’ll be at thy elbow. It makes us, or it mars us. Think on that, And fix most firm thy resolution.
Be near at hand, I may miscarry in ’t.
Cassio has just entered and the plan him and Iago talked about before is just about to take place.
I know his gait, ’tis he.—Villain, thou diest!
That thrust had been mine enemy indeed But that my coat is better than thou know’st I will make proof of thine.
Roderigo misses his only shot and Cassio pulls out his sword to go and stab him. Then Iago comes from behind to finish the plan.
Oh, I am slain!
'Tis he. Oh, brave Iago, honest and just, That hast such noble sense of thy friend’s wrong!
Othello comes onto the scene and all he sees is that Iago just saved Roderigo from being murdered.
Cassio comes back and is yelling out for help asking if there is a guard around, while Iago and Roderigo still act as if he is the villain.
Oh, wretched villain!
What, ho! No watch? No passage? Murder! Murder!
Iago comes back out of nowhere to act as if he just got there and has no idea what is going on.
Who’s there? Whose noise is this that ones on murder?