How are the themes of witchcraft and rhetorical power portrayed in the play in relation to the main characters?

In terms of rhetorical power,
Iago uses imagery to twist what is said or understood. For example,
time after time, he allows Othello to believe something other than reality by
introducing a hypothetical situation or failing to use names. Other times he flat out
lies. For example in Act iv.i, he lets Othello imagine a "kiss in private" and being
"naked in bed with a friend". Both of these connote Desdemona and Cassio, but neither
name is ever mentioned. Other rhetorical devices that Iago uses include
puns and rhetorical questions. One line
captures both of these:

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Lie - with her, on her, what you
will.

Iago is playing with
the word "lie" to mean sex here in Othello's mind. Ironically, we audience members know
that it isn't true and an untruth is a lie. He leaves the end of the comment "what you
will" meaning that Othello can believe whatever he wants or can imagine. Iago is
purposely being non-chalant to let Othello get worked up about this affair that is
actually not happening between his wife and Cassio.

The
theme of witchcraft also comes through in this same scene.
Iago is thought to be a representative of the Devil and the religion or practice of the
Devil would certainly be witchcraft. Here, after causing Othello's mind to run so wild
that he falls into an epileptic siezure, Iago demonstrates witchcraft in two
ways:

My
medicine, work! Thus credulous fools are caught:
And many worthy and chaste
dames even thus,
All guiltless, meet
reproach.

First, he notes the
work he has done as "medicine". One of the first images people experience in thinking
about witchcraft has to do with a big black cauldron or spells being cast.
Medicine is an allusion to each of these. Then, his work is to
defeat the righteous. The words credulous, worthy, and
chaste all speak to goodness, the enemy of
evil.