Roderigo, Cassio and Othello are all manipulated by Iago, but how are they different? How do the three comment on each other as characters? In...

Iago is the spider who weaves his web that will "ensnare them all."  But the methods Iago uses for each of these three characters are indicative of their intellect, integrity, and passions.  For Roderigo, Iago appeals to his sexual desire to obtain Desdemona for his own.  Roderigo is easily manipulated.  All Iago has to do is mention Desdemona as bait, and Roderigo will do whatever Iago asks:  sell his lands, give Iago more money, follow Iago to Cyprus, pick a fight with Cassio.  Iago's main method of persuasion with Roderigo is repetition:  "Put money in thy purse," Iago tells Roderigo over and over again.

Cassio is more of a challenge for Iago.  Cassio is a courtier with good looks, fine manners, and a good education.  Cassio's weakness is that he cannot hold his licquor.  So, Iago gets him drunk, and sends Roderigo to pick a fight with him.  This fight has consequences beyond Iago's wildest dreams, ending with Cassio wounding Montano and losing his position as lieutenant.  This fight took some maneuvering, but Iago was quite successful in bringing Cassio down.  The second part of his plan, to make Cassio seem like Desdemona's lover, required much cleverness.  When Cassio turns to him for advice, Iago tells him to go to Desdemona to get his position back, and Iago will, of course, make this innocent conversation seem like something else.  If Cassio were not the conscientious soldier that he is, or if Cassio had been unwilling to accept the responsibility of his own drunken actions, or if Cassio had been less loyal to Othello, Iago's plan would not have worked.  Cassio is indeed a "great fly," caught in Iago's net.

With Othello, Iago must pull out all the stops.  Othello is much more difficult to manipulate than either Roderigo or Cassio.  Othello is powerful, honorable, deeply in love, "not easily made jealous."  Iago resorts to suggestion, innuendo, reverse psychology, staging, oaths of loyalty, "ocular proof" (the handkerchief), racial and gender stereotyping.   Iago's manipulation of Othello comprises most of Act 3 and part of Act 4.  Othello does not fall easily, demanding that Iago "prove his love a whore."  In this way, Othello is clearly a more complex and intriguing character than Cassio or Roderigo.