The way that Shakespeare introduces the characters is very clever indeed. The opening lines immediately convey the atmosphere (of mystery and intrigue) surrounding the conversation between Roderigo and Iago, the very different characters of the two speakers, the situation at the beginning of the story and the relationship between the two of them.
The subject of our attention is to be the character of these two individuals and how their unique characters are exposed in the opening act and scene of Othello.
The impatience (and immaturity) of Roderigo immediately becomes apparent with the use of the phrase Never tell me' in the first line. His immaturity is hinted at with the tone of the language that he is using. He sounds like a child with this use of language.
The reference to his (Roderigo's) purse' tells us of the relationship between the two. .
Iago immediately exerts control over the situation putting Roderigo down with Sblood, but you'll not hear me!' a show of the fact that Iago believes Roderigo to be a nave (and immature) fool that is to be manipulated. He batters back at Roderigo's attack with his own offensive (before Roderigo's attack gains momentum). This is a good show of the fact that Iago is a confident character that has a very good grasp of the situation. .
Iago's arrogance is further hinted at with I know my price' suggesting that he has a very high opinion of himself. He is not afraid of asserting this now that he has quietened Roderigo. He feels that his merit is unrewarded and this is an important issue to consider when looking upon his attitude and actions.
We also learn that Iago has some very influential friends in Venice and we discover this in line 8 - 9 with Three great ones of the city, In personal suit to make me his lieutenant'. .
Feelings that we were not first aware of come to the foreground in line 15 - 29 where we discover that he despises Cassio as he does not believe him to be worthy of the title Lieutenant'.