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:
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.