What occurs in "Romeo and Juliet" before Act III, Scene 1? And how does the early part of the play build up to Act 3, Scene 1?

Romeo and Juliet is a beautifully balanced, poetic play:  The first two acts contain the exposition, introduction of main characters, and the rising action with its presentation of the conflicts between youth and age and between opposing families, which, then, lead to the climax of Act 3. 

The juxtaposition of characters is an integral part of Romeo and Juliet's first two acts which underscore much of the meaning of Act 3.  In Act 1 peaceful Benvolio acts as a foil to the belligerent Tybalt who later appears enraged in Act 3; Juliet warns against the haste of a relationship with Romeo in Act 2, foreshadowing the tragedy of their love.  Also, in his soliloquy, Act 2, Sc. 2, Friar Laurence ponders the juxtaposition of virtue and vice and the dangers of excess.

Another juxtaposition is in the placing of light/dark imagery in the first two acts.  In the dark, which will be his and Juliet's safety, Romeo speaks of Juliet as light:  "she doth teach the torches to burn bright!" (Act 1, Sc. 5) and "It is the east and Juliet is the sun!" (Act 2, Sc. 2).  The danger comes, later, in the light beginning with the swordfight of Tybalt/Mercutio.

In Act 3, Tybalt and Mercutio--both characters of excess, one of ire, one of mischief--are juxtaposed as enemies whose hatred climaxes as a duel.