Who is he? Mercutio is one of Romeo's best friends, (the other being Benvolio) and also related to the Prince Escalus.
Mercutio first appears with the 'maskers' in Act 1 Scene 4, when he delivers his 'Queen Mab' speech and persuades Romeo to take part in the dance/party and renounce his idea of being merely a torchbearer. He next appears with Benvolio, in search of Romeo after the party.
The following morning, the same two are again in search of Romeo, who at last stumbles in and jests with them, when the Nurse (to Juliet) enters, the jesting continues at her expense.
He last appears during Act 3 Scene 1, the afternoon of the day that he is killed.
As in both the Luhrman and Zeffirelli films, the time is the same as in Act 2 Scene 6 but the scene has shifted from the intimacy of the priest cell to a public place, and Benvolio's opening words 'spell' trouble.
("I pray thee, good Mercutio, let's retire.
The day is hot, the Capulet's abroad,
And if we meet we shall not 'scape a brawl;
For now, these hot days, is the mad blood stirring.)
Mercutio, Benvolio and their followers are in the public square of the city. The two, as usual, are frolicking around. Benvolio advises Mercutio that they both go home since the day is hot and the Capulet's may be abroad. There is sure to be to be trouble if they meet. Mercutio, always ready for a fight, charges his friend as being one of those men always talking about peace, but ready to fight on the slightest occasion.
("...Thou! Why, thou wilt quarrel with a man that hath a hair more or a hair less in his beard then thou hast. Thou...