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

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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. For example, after he has convinced Roderigo to help him, he says 'though he in a fertile climate dwell, plague him with flies'; through the use of a humble metaphor Iago menages to make himself understood. On the other side, Iago uses his eloquence to keep Brabantio on his toes: saying 'for shame, put on your gown; your heart is burst, you have lost half your soul' he menages to alert Desdemona's father for the bad break he will hear; The news about Desdemona and the Moor is given by Iago using a metaphor 'an old black ram is tupping your white ewe' and connoting negatively the piece of information using an humble metaphor.

Is interesting to notice that Iago 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 that he doesn't know anything about her 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'), always behaving as an 'honest knaves' (the reader should not forget about Iago's staement in the first scene: 'in folowing him, I follow but myself'); 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'.

Iago's influence is proved by the third scene that sets in the Council house: his conditioning against the characters is so powerful that Desdemona and Othello's marriage becomes a business of the state, just like the war in Cyprus.