Both Acts 1 and 2 are central to the development of the plot. Plots usually have five parts to their structure, three of which are the exposition, the rising action, and the climax. The exposition is the moment at the very beginning of the story in which we are introduced to the characters, the setting, the main conflict, and also given relevant information pertaining to the characters and their background. The climax is the most emotionally tense moment of the story, the moment in which the conflict's resolution is in sight and becomes inevitable, while the rising action is all action leading to the climax. Act 1 serves as the exposition, while Act 2 serves as the plot's rising action.
In Act 1, we are introduced to all of the central characters, including the Capulets, the Montagues, and even the play's tragic hero, Romeo. In the Prologue of the first act, we are even told of the longstanding feud and that the two noble families "[f]rom acient grudge break to new mutiny" (Prologue.3). Hence, we are told about one of the central conflicts, which is that the two families are warring with each other and that central conflict helps create the character vs. fate conflict that leads to both Romeo's and Juliet's deaths. In the first scene, we are especially introduced to the characterizations of central characters, especially Romeo's lovelorn, rash, emotionally driven mind. In the second and third scenes of the first act, we are introduced to our heroine Juliet and given a hint concerning another conflict, which is the prospect that Juliet may become engaged to Paris. In the final scene of the act, we witness the hero and heroine meet under strenuous circumstances, showing us the emergence of the character vs. fate conflict, as well as the character vs. character conflict we see through Tybalt's feelings of anger and insult at Romeo's presence at the Capulet ball. Since all of these serve to introduce characters as well as set up the conflict, we see that Shakespeare's purpose was clearly to use the first act as the exposition.
There are several ways in which the second act is used to create the rising action leading towards the climax. Rising action can first be seen in the second scene of Act 2 in which Romeo, upon threat of death, sneaks into the Capulet's orchard to see Juliet again and there they make vows of love, promising to be married. It is also in Act 2 that we meet Friar Laurence who, against his better judgement, agrees to secretly marry the couple. We see his hesitations to marry the couple in his lines, "So smile the heavens upon this holy act / That after-hours with sorrow chide us not!" (II.vi.1-2). The secret marriage serves as rising action leading to the final climax of character vs. fate because the secrecy of the marriage creates more problems, including the later need to fake Juliet's death to help her escape being forced to marry Paris. It is her faked death that partially helps lead to her real death, as well as Romeo's. Finally, it is also in this act we learn that Tybalt has challenged Romeo to a duel, which is rising action leading to the climax of the character vs. character conflict, which leads to several deaths and Romeo's banishment, also eventually leading to his own death.
Hence we see that Acts 1 and 2 are central for introducing the characters and conflicts as well as creating the rising action leading to the climax.