In Othello how does the way Shakespeare begin the play immediately involve the audience?

The play begins in medias res with a somewhat heated discussion between Iago and Roderigo. We don't know exactly what the discussion is about, but we do know that there is obvious tension between the two men as Roderigo is angry with Iago that he did not reveal some information.  Iago, more placating, tells Roderigo that they share a mutual dislike for the same man.  The audience is immediately engaged.  We'd are eavesdropping on what might escalate to something bigger, and we are curious as to the source of their disagreement.  Iago goes on to reveal that the pompous Othello passed him over for a promotion when he clearly was more experienced and more deserving of the position of lieutenant than the inexperienced, youthful Cassio.  Our sympathies are somewhat aroused, since we all know what it is like to be placed second to someone we consider our inferior.

Later Iago reveals that he plans to get revenge on Othello: "I follow him to serve my turn upon him."  And later he declares, "I am not what I am."  As we begin to learn how Iago thinks, we are begin to readjust our opinions of both Iago and Othello.  Iago, we realize, is crafty and vindictive, and he will cause trouble for his general.

If the audience is not engaged at this point, Iago's plan to wake up Brabantio, Desdemona's father, and tell him of Othello and Desdemona's elopement will do the trick.  Iago, shouting, uses crude and vulgar images of Desdemona and Othello sleeping together:  "a Barberry horse is covering your white ewe," and "making the beast with two backs." And the audience is able at this point to piece together the reason behind Roderigo's anger.  He wanted Desdemona for himself.  Brabantio is so enraged that he gathers a small army of men to confront Othello.

The tension in the very first scene escalates from a small argument between two disgruntled men to a force of men raising swords against Othello.  We are fed just enough information to keep us wanting to know more, and as our respect for Iago's intelligence grows, our opinion of his moral character declines, and if Iago can cause such chaos in the first scene, we wait in anticipation to see what havoc he will cause in the play as a whole.