Benvolio is generally a stabilizing force. He tries to avoid fights. In this scene, he is not successful.
In Act 3, Scene 1, Benvolio wants to go inside because it is hot. He really wants to go inside to avoid any fights. Mercutio teases him, thinking that he really does want to start a fight.
An I were so apt to quarrel as thou art, any man
should buy the fee simple of my life for an hour and a
quarter. (Act 3, Scene 1, p. 61)
Mercutio thinks it’s fun to talk about fighting, and Benvolio enters the conversation in a witty and good natured way.
Benvolio tries to warn Mercutio that Tybalt, a Capulet, has come in. Mercutio says he doesn’t care, but there is soon an altercation. Benvolio tries to stop it.
We talk here in the public haunt of men.
Either withdraw unto some private place(50)
And reason coldly of your grievances,
Or else depart. Here all eyes gaze on us. (Act 3, Scene 1, p. 61)
It doesn’t work. Soon, Tybalt and Mercutio are fighting and Romeo tries to break up the fight. His intervention causes Tybalt to kill Mercutio, and Romeo kills Tybalt. This causes Romeo’s banishment.
In William Shakespeare’s Romeo and Juliet Benvolio appears throughout act 3, scene 1. Early in the scene he tries to prevent violence from occurring between Mercutio and Tybalt. Once Mercutio is killed, his function changes somewhat. He is the character that warns Romeo that Tybalt is returning. When Romeo vengefully kills Tybalt, he advises him to escape to save his own skin:
Romeo, away, begone!
The citizens are up, and Tybalt slain.
Stand not amazed. The Prince will doom thee death
If thou art taken. Hence, begone, away.
Once the Prince arrives, Benvolio’s function changes to that of explainer. He tells the Prince what happened and how Tybalt and Mercutio were killed. Perhaps it is surprising that he does not try to deflect blame from Romeo as he accurately describes Romeo’s involvement, saying that he:
Had newly entertained revenge.
By using the character of Benvolio this way, Shakespeare is able to clearly present the plot situation to the audience. They know that Romeo has fled for his life and that he will be a marked man in Verona. This establishes the idea that he cannot just stroll back into town later on to be with Juliet. Instead they must devise a secret plan, which goes awry, resulting in the young lovers’ deaths.