Othello has appointed Cassio to the job that Iago wanted. It is Iago’s ultimate plan to bring about the downfall of Othello, but he’s not above ruining Cassio’s career at the same time. In Act 2 Scene 3, Iago gets Cassio drunk and then plants the idea in Governor Montano’s head that Cassio is an alcoholic, and that he worries about the trust Othello has put in him:
Iago You see this fellow that is gone before;
He is a soldier fit to stand by Caesar
And give direction: and do but see his vice;
‘Tis to his virtue a just equinox,
The one as long as the other: ’tis pity of him.
I fear the trust Othello puts him in.
On some odd time of his infirmity,
Will shake this island.
Montano But is he often thus?
Iago ‘Tis evermore the prologue to his sleep:
He’ll watch the horologe a double set,
If drink rock not his cradle.
(Literally, Iago is saying “You’ve seen the virtues the man has to offer, but now you realize he’s got just as many vices.” He then goes on to suggest that Cassio drinks himself to sleep every night.)
The truth of the situation is that Cassio is a lightweight drinker and he knows it. When Iago first offers him wine he responds , “I have very poor and unhappy brains for drinking” and “I have drunk but one cup to-night, and…
dare not task my weakness with any more.” What Cassio does not realize is that you can’t tell Iago something like that. He’s going to use it against you.