Reading Iago's first cue, the reader is captured by his speaking skills; first of all the reader notices that the character's speech is written in lines: in the reader's mind, this choice creates the idea of a skilfull and intelligent character.

Going on reading the reader's conjecture is confirmed, but since he finds words full of hate and jealousy he may also conjecture that Iago may use his skills for evil porpouses. Indeed Iago is angry with his general because he had chosen Michael Cassio as his liutenant insted of him, so Iago connotes his general as a man who loves only 'his own pride and purpouse' and Cassio as an incompetent who doesn't know 'more than a spinster' about the division of a battle.

So the retorical skills of the character (who can also focus the reader's attention on specific passages leaving irrelevant details - 'and in coclusion'- and using rethorical devices -'and I..' 'and I..') gain a negative connotation, since they are associated to presumption and jealousy.

In addition, considering the whole first act, the reader may recognize the signs of Iago's deceit. Right form the first cue, Iago uses expressions of the field of intimate reletionship (wife, spinser): the choice prepares the reader's mind for Iago's revenge that takes place thanks to the unreal reletionship between Othello's wife and Cassio.

In order to build up the deceit he ivolves different characters (Roderigo, Brabanzio) using his rethoric skills and intelligence. In addition, he shows his knowledge about human beings, that's what allows him to manipulate other characters for his own porpouse; Iago knows that Roderigo hates Othello because he's in love with his wife and that Brabantio thinks his daughter's still as fragile as a baby and doesn't know anything about hers marriage: for this reason both characters become pawns of his plan. 

Iago plots his deceit also instilling doubt in Othello's mind and teasing him ('nay, but he prated and spoke such scurvy..', 'you were best go in'); Othello says that Desdemona's love is his 'fortune' and, knowing that, Iago creates right from the beginning the setting for his revenge using Cassio who 'hath a person and a smooth dispose to be suspected' and playing on Othello itself who 'will as tenderly be led by th'nose as asses are'.