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.
– William Shakespeare
Othello, Act 2, Scene 3. Iago is full of praise for Cassio’s skills as a soldier to Montano. But his compliment is a double-edged one. In the same breath he tells the Cyprus Governor that Cassio is as great a drunk as he is a soldier and his drunkenness may cause major problems for Othello in Cyprus. Iago goes on to tell Montano that Cassio gets drunk every night. However, he is lying and knows that Cassio is not a regular drunk, but overindulged on this occasion because Iago manipulated him into doing so. Iago is also manipulating Montano here in order to damage Cassio’s reputation, so that he can take over Cassio’s job as Othello’s lieutenant.