But I, for mere suspicion in that kind,Will do as if for surety. He holds me well
In this case, Iago is describing the effect and power he holds on others, by spreading suspicion. Could it be that he himself is entranced by the workings of suspicion, played by some external force? His power seems to work against him!
Demand me nothing. What you know, you know.From this time forth I never will speak word.
His last rebellion, his final influence over the situation -- knowing nothing. And its ironic because he was the source of all knowledge and information that sparked all the events, and now that everything is done, he is still. There is no more movement, even if they would like there to be some. In this way he is really like Shakespeare, having the power to cause and inhibit action through knowledge -- the greed of which is Othello's fatal flaw.