Iago takes advantage of Cassio’s trusting nature by leading him into troubles while pretending to be his friend.
At first, Iago pressures Cassio to drink, knowing that Cassio can not tolerate lots of alcohol: “If I can fasten but one cup upon him / With that which he hath drunk tonight already, / He’ll be as full of quarrel and offence / As my young mistress’ dog.” (II. iii, 45-48). It is established that Cassio lacks alcohol tolerance. Additionally, this is the first step Iago takes to strip Cassio of his position. It is shown that Iago is extremely aware of people’s weaknesses and their characters, whereas other character knows about him.
Iago not only makes Cassio drunk but also lies to Montano that Cassio needs to drink every night to fall asleep, “‘Tis evermore the prologue to his sleep.” (II. iii, 123) As a result of this quotation, Montano fights with Cassio defending Roderigo who intentionally made Cassio angry. Consequently, Othello withdraws Cassio from the position of lieutenant.
Despite of the trouble caused by Iago, Cassio still seeks advice from him on how to get back his position of lieutenant and Othello’s trust. Iago advises Cassio to, “Confess yourself freely to her,/ importune her help to put you in your place again./……..
/This broken joint between / you and her husband entreat her to splinter, and, my / fortunes against any lay worth naming, this crack of / your love shall grow stronger than it was before.” (II. iii, 307-308, 311-314) He advises Cassio to talk to Desdemona about influencing Othello.
Although this might appear to be a great advice, but it is a form of manipulation. Since Iago abuses Cassio and Desdemona’s relationship to build an extreme jealousy in Othello’s view. Overall, Iago manipulates Cassio using his trusting nature and by appearing to be his friend.