1 {scene: Hospital waiting room}
1 Ophelia: It seems so pointless, Mercutio. There's no reason behind what's happened. If this was a story I was reading, I'd be disappointed.
2 Ophelia: It's just completely out of the blue. A cheap shot. Drama for its own sake and nothing else.
3 Mercutio: Life's but a walking shadow, a poor player that struts and frets his hour upon the stage and then is heard no more: it is a tale told by an idiot, full of sound and fury, signifying nothing.
4 Ophelia: Don't try to justify what's happening here!

Macbeth, Act 5, Scene V. Dunsinane Castle.

Macbeth

She should have died hereafter;
There would have been a time for such a word.
To-morrow, and to-morrow, and to-morrow,
Creeps in this petty pace from day to day
To the last syllable of recorded time,
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.

Macbeth recites this soliloquy after learning of his wife's untimely death. At this point, very near the end of the play, and after witnessing so much death and tragedy around him, it is enough to cause Macbeth to despair that life is ultimately meaningless and wish for death himself.

Mercutio isn't quite using it in the same context or tone here. He's merely pointing out that the vagaries of life we must all live with have no rhyme nor reason. Our lives are not scripted by some writer, so there is nothing to prevent arbitrary events from interfering with our plans and desires.

---

2018-08-04 Rerun commentary: We haven't broken the fourth wall here, we're just kind of leaning on it a bit.

Presumably Dunsinane Castle has four very solid walls.