I have changed this to Act I, as Mercutio does not appear in the first scene. Benvolio, who does appear in scene 1, comes across as a pragmatist: he tries to avoid a fight with the hot-blooded Tybalt, although he fights him when he has no choice. Benvolio is a problem-solver: he tells Romeo's father that he will find out why Romeo is mooning about. When he discovers it's because of unrequited love for a woman who insists on staying chaste, Benvolio takes a practical approach, telling Romeo there are plenty of other fish in the sea. While Romeo throws himself body and soul into love, the down-to-earth Benvolio advises him to "examine other beauties." Benvolio is invested in his world and the people around him, a social being, but not one to get too worked up into unnecessary passions.
Mercutio appears in scene four. He's the poet and wordsmith: he has a way with words, revels in wordplay and willingly engages in debating about love with Romeo. Mercutio's high-spirited personality will have none of Romeo's lovesick mooning. Instead, he tells Romeo that if love is rough on him, he should be rough on love. Mercutio shows his impatience, urging them twice to get on to the party. Finally, he's the storyteller, the person who keeps others entertained, as he does with a long, fanciful tale about Queen Mab, the fairy queen, as they head to the Capulets'. He shows his imaginative spirit as well as his way with words as he weaves a detailed picture of the tiny Queen with her whip of cricket's bone and her wagon driver as a gray-coated gnat. If Benvolio is the prosaic, steady friend, Mercutio is the quick-witted life of the party who it's hard not to love.