Othello 2.3 L. 41-278

The scene jumps right into Iago's plan with Iago attempting to get Cassio drunk. Cassio has a low alcohol tolerance so he is done after one drink, but with all the festivities going on Cassio eventually gets caught up in the atmosphere and becomes drunk. There is the reintroduction Montano in this scene and he acts as one of the sober gentlemen. So Cassio becomes drunk and he leaves to do his guard duty but Iago manipulates Montano. He plants the idea that Cassio is a drunkard who is always drinking. Montano is skeptical until Cassio comes in chasing Roderigo while shouting obscenities. Montano attempts to break them apart but eventually is caught up in the fight as well. While this is happening, Iago has Roderigo go ring the city bell to wake everyone, including Othello. Eventually Othello comes and is able break up the fight. He has Iago tell him the story of what happened and Iago obviously point the finger to Cassio. Othello loses his temper and fires Cassio, exactly as Iago planned.

Some of Cassio's characteristics are quite unique compared to the other characters. For example, Cassio is able to stay calm even when Iago was blaming him. Usually, someone would try to defend themselves when they are being accused for something, but Cassio takes the blame and knows that he deserves what he was going to get. Cassio is also quite naive. He takes information that he gets at face value and never questions it. Iago was throwing him under the bus and he still believed that Iago genuinely cared for him. 

Why doesn't anyone suspect Iago for starting the fight? Especially with the things he said to Montano.
I think it is mostly due to Iago's reputation. Iago is well known for being an honest person, as ironic as that is. Because of his reputation, Iago is trusted and Montano doesn't doubt him. Iago also isn't doubted because he isn't shown to have any relationship to Roderigo, the culprit. 


https://blogs.glowscotland.org.uk/ab/B106-Othello/files/2012/11/02_03.jpg

This image of Iago comforting Cassio shows Iago and Cassio's personality. Iago is looking at Cassio with a condesending attitude but Cassio is very sad about him being fired.