In Othello, explain the contrast in the way Iago speaks to Brabantio and the way Roderigo speaks to him.
The way in which Iago addresses Brabantio stands completely in contrast to Roderigo's measured language, addressing Brabantio respectfully as "Signior" even in his initial yell to catch Brabantio's attention. Roderigo, we may infer, knows (or suspects) from the outset that Brabantio will recognize his voice. Iago, on the other hand, uses Brabantio's name without an honorific title ("what, ho, Brabantio!") and repeatedly yells "thieves! thieves! thieves!" in such a way as to alarm him.
Later, the language Iago uses to describe the marriage of Desdemona to Othello is very coarse: "an old black ram / Is topping your white ewe." Roderigo then attempts to temper this: "Most reverend signior, do you know my voice?" He is seemingly attempting to calm Brabantio by indicating that he, Roderigo, is a person known to him; when Brabantio declares, "the worser welcome" to this news, Roderigo continues his efforts at placating him: "Sir, sir, sir," "Patience, good sir."
Iago, repeatedly returning to coarse sexual imagery ("you'll have your daughter covered with a Barbary horse") and cursing ("'zounds, sir") is rightly described by Brabantio as a "profane wretch." Probably, Iago feels free to address Brabantio in this way because, unlike Roderigo, he is not already known to Brabantio and does not feel the need to make any particular impression.
If you're speaking of the opening act, then there's a very distinct difference between how Iago speaks to Brabantio and how Roderigo does. We must assume that Iago is hidden from sight and that Brabantio cannot see him. Iago is crude, rude, and very aggressive in his speech to Brabantio. Iago says
'Zounds, sir, you're robb'd! For shame, put on your
Your heart is burst, you have lost half your soul;
Even now, now, very now, an old black ram
Is tupping your white ewe. Arise, arise!(95)
Awake the snorting citizens with the bell,
Or else the devil will make a grandsire of you
Iago is much more aggressive in his speech and tone because he wants to remain, in appearance at least, very loyal to the moor. Iago can't do that if he's recognized to be rousing people against his marriage to Desdemona. Roderigo is much kinder to Brabantio and is a familiar, recognized, former suitor of Desdemona's.