Well, right from the very start, you have the chorus giving away the ending of the play with "a pair of star-crossed lovers take their life". The audience is told that they will be seeing a play about two feuding families who hated each other so much for a very long time that it took the death of their two children to end the fighting. This leaves the audience with "How did all this happen?" and "Why did they kill themselves?" going through their minds before Samson and Gregory even make their entrance. This prologue is the same as any television prologue that gives the background of the plot, introduces the main characters, and foreshadows main events, leaving the audience to sit back and enjoy how this all unfolds. Just look at "The Ballad of Gilligan's Island", "The Fresh Prince of Bel Air", "The Beverly Hillbillies", or "The Brady Bunch". In each prologue you get characters, main conflict, and foreshadowing. Shakespeare's prologue is no different except that it's written as a sonnet. I actually have my students choose a TV show that doesn't already have a written prologue and create one. I just collected about 15 prologues for "Lost" and at least a dozen each for "American Idol" and "House".
This is a very subjective question. What I mean is that the audience reaction depends on how the director chooses to interpret the opening scenes. For instance, in the movie version directed by Zeferelli, the opening scene takes place in the streets and encompasses everyone on the streets that morning. The Capulet and Montague factions create a near-riot, and the Prince nearly explodes in anger at the chaos they keep creating.
You can't do that on such a scale on a stage, but you can create the same kind of chaotic atmosphere by the way Sampson and Gregory wish for a confrontation and say what they will do if they do encounter a Montague--and then having that confrontation. The audience knows right from the start that trouble is brewing and this is not going to be a light-hearted comedy.