How often do you remember watching a play in which a character announces he has to go to the bathroom, and the action stops until he has done so? Not too often. Why? Because plays take place in a highly edited form of real time, and there's not a moment to waste. So why do many inexperienced writers waste so much time on characters saying "hello" and "goodbye"? Instead, let each scene begin as late as possible in the action and end as early as possible. If two characters are sitting together talking, we can assume at some point they said hello. If one or both show up later in another scene, we can assume they said goodbye. The only time it's useful to have a "hello" of some kind is if the greeting itself is part of the dramatic action, or if the audience hasn't met the characters before and we need the information the greeting will give us. Otherwise, cut to the chase and start the scene in the middle of the action.
The same thing is true of the entire play. It's often tempting to start early in the story. For example, in my play Just Add Zombies, we could back the play up to early in the rehearsal process, show how bad the actors are, maybe even show Aunt Lillian casting Mitchy as Romeo because he's her nephew. But that's not what the play is about, and ultimately starting this early would be a disservice to the play; we can learn these pieces of information with just a few lines of dialogue (or perhaps a well-placed monologue), and it's far more exciting to place that exposition (i.e. information) in the middle of the action. In other words, start late.
You may also recall that in a previous blog, I talked about the "question of the play," that question to which the audience is waiting for an answer so they can go home satisfied. In Romeo and Juliet, I noted, the question is "will they stay together?" just as in Hamlet it's "will he get revenge?" Once the question is answered, wrap it up and get out. In other words, end early.