This scene is really important, because it influences everything that happens afterwards - like Romeo being exiled, Juliet taking the potion to avoid marriage etc.Anyway, the scene starts with talk of hot tempers and introduces the idea that a fight is going to happen. Mercutio is clearly in a bad mood, because he's arguing with Benvolio, so the audience can see he's not going to back down and walk away from a fight. Already tension is getting built up, so the audience would be thinking what's going to happen now?We've been told in Act 2 Scene 5 that Tybalt sent a letter to Romeo's house challenging him to a fight and as soon as Tybalt enters he asks where Romeo is, letting the audience know there's going to be a fight. Again, a build up of tension, with the audience now waiting for the fight.When Romeo won't fight the audience might think danger has been averted, then Mercutio steps in and that builds up tension because you start to wonder what's going to happen.After the fight bewteen Mercutio and Tybalt Mercutio is still joking, so the audience would think everything was going to be ok. Then comes the shock "brave Mercutio is dead". That heightens tension and then Romeo loses his temper and kills Tybalt.Shakespeare tends to push up the tension with talk of a fight, then push down the tension with a joke. Then he pushes it up again, which increases the overall tension.