Act 1 does a lot of character-establishing work quickly:

• Romeo begins the play as a melancholy, self-absorbed young man mourning a love that was never returned. His rapid switch from Rosaline to Juliet is often read as evidence of his impulsiveness — a trait that drives the tragedy later.
• Juliet is presented as obedient and untested at first — not yet 14, guided by her mother and the Nurse — which makes her later boldness and independence more striking by contrast.
• Mercutio is witty, cynical about love, and physically volatile — a contrast to Romeo’s romanticism.
• Tybalt is established purely through aggression: he recognizes Romeo’s voice at the feast and wants to fight him immediately, setting up the conflict that erupts in Act 3.
• The Nurse is Juliet’s confidante, warm and talkative, often providing comic relief and a contrast to the formality of Lady Capulet.