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.
Act III, Scene i is the key turning point of the play as it contains the event which twists the plot from comedy to tragedy. To give you a brief summary of what's happened until III, i starts (and note how it starts - with oppressive, ominous heat!), Shakespeare has sketched the lives and feelings of both Romeo and Juliet (in separate scenes) and the atmosphere of the Montague boys' friendships.
The Capulet party (Act I, Scene 5) sees Romeo and Juliet meet and fall in love - and also sees Tybalt, recognising a Montague's voice (from behind a mask), swear to be revenged on Romeo. Romeo and Juliet meet (in the so-called "balcony" scene) and vow to marry. Romeo then goes to Friar Laurence, who agrees to marry them: and he does, the same day (in the scene previous to Act 3, Scene 1).
In short, everything is going well: the lovers have met, fallen in love, and look set to be married, which will unite their households (so Friar Laurence thinks). And recently critics have argued that even the opening scene does not present a seriously violent threat: it is largely comic, and (like the opening scene of The Comedy of Errors) its violence never recurs in the same way.
It's not so much that the first half of the play builds up to Act 3, Scene 1: but it carefully puts all the pieces in place so that Act 3, Scene 1 can blow them apart. At the end of it, Mercutio and Tybalt are dead - and Romeo is a murderer.
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Romeo and Juliet are married by Friar Lawrence. Acts I and II prepare the audience for their wedding by having Romeo court Juliet in the famous balcony scene (Act II, Scene 2), Juliet imply that Romeo should ask her to marry him, Romeo's visit to the Friar to request that Friar Lawrence perform the ritual and Friar's warning "they stumble that run fast," and finally Romeo's conversation with Juliet's Nurse to make arrangements for the wedding.
After having met in act I, act II has the pivotal balcony scene where their love in declared despite the family's feelings for one another. After they have decided they "love" one another, Juliet tells Romeo, she will send her nurse to him the next day to decide when and where they will marry. Romeo runs to the Friar to tell him he is no longer in love with Rosaline, but Juliet. Although the friar thinks he has rushed into this, he decides to marry them thinking this could cure the families' difficulties. Their marriage is what causes the intensity in Act III as Tybalt wants to fight Romeo, but Romeo can no longer be angry with Tybalt as he is now family.