How does Iago in Othello mislead his listeners with his "truths"?

Iago is able to mislead and misdirect his listeners
because he understands what the people around him need to hear.  Iago is brilliant in
understanding character motivation.  No other character in the play is so attune to
context and individual motivation.  Iago is able to use circumstance and character to
fully ensure that his listeners do what he wishes them to do without even knowing it. 
It is through this insight into people that he is able to manipulate others into doing
his bidding.  The manner in which Iago is constructed is done so without any long term
goal or vision.  Rather, Iago is just a force of malevolence or one that is perpetually
angry.  It is such a condition that allows him to perfectly design where individuals
should be in order to be the most unhappy, matching his own personal state.  In this
light, Iago is able to mislead others, pretending to provide truth, but enabling his own
ends of transgression to be met.  We see throughout the play.   For example, Iago uses
the vision Roderigo has of a union with Desdemona to manipulate Roderigo. Cassio is a
man driven by the need to maintain outer appearances, and he easily accepts Iago's
advice that he recover his rank by going through Desdemona. Iago also uses to his
advantage the fact that Desdemona is of a kind and generous nature, one who will gladly
accept the opportunity to persuade her husband to make amends with his lieutenant. And,
finally, Iago uses Othello's jealous nature and his apparent insecurity to convince
Othello of Desdemona's infidelity.