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.