Iago's use of prose vs. poetry

In William Shakespeare's Othello, the villain Iago often switches from speaking poetry, normal iambic pentameter, to speaking prose. Initially, when asked to make a conjecture as to why he does so, I thought that Iago switched to prose when he "lost his cool." I thought that the less structured nature of the prose was representative of flaws in Iago's attempts to deceive. However, having finished the play, I now have a new more specific, and hopefully accurate, theory. I believe that Iago speaks in prose when he is plotting, mainly with Roderigo, and in poetry when he is carrying out his plot, or deceiving others. Because Iago is actively deceiving others for the majority of the play, he is most often speaking in iambic pentameter. Therefore, it is important to notice the times when he switches to prose. For instance, in Act 4, Iago says to Roderigo in prose, "....But, Roderigo thou hast that in thee indeed which I have greater reason to believe now than ever-I mean purpose, courage, and valor-this night show it. If thou the next night following enjoy not Desdemona, take me from this world with treachery and devise engines for my life" (IV.ii.207-211). Here, Iago is plotting to have Roderigo kill Cassio-an integral part of his plan. He has full knowledge that Roderigo will be the murderer. However, after the murder attempt takes place, Iago says of Bianca to the Venetian men, "Gentlemen all, I do suspect this trash/To be a party in this injury/....Roderigo? No-yes, sure. Oh Heaven! Roderigo" (IV.ii.85-90). Iago says this in iambic pentameter; he is clearly deceiving the Venetian men because he is aware that Bianca was not the murderer and he offers false surprise at Roderigo's presence at the scene.