Symbolism, Imagery, Allegory
Drama, King
Perhaps you were unaware, but all the world's a stage, folks.
The first use of this metaphor occurs in Book 3, when Marcus imagines a man who has changed his wicked ways and is at peace with himself. Such a man, Marcus says, will not be unpleasantly surprised when the end of his life overtakes him:
Fate does not catch him with his life unfulfilled, as one might speak of an actor leaving the stage before his part is finished and the play is over. (3.8)
Marcus's point is that a man who creates an independent mind through the constant application of philosophical principles will have no fear that he hasn't "played his part" as a good citizen of the universe, no matter how short his life might be.
At the end of the work, Marcus does a nifty thing by keying right back into his earlier use of the theater metaphor. It's actually so similar to the earlier use that a rational being might suspect that both were written at the same time.
As Marcus pens his envoy—or farewell postscript—he brings back the acting metaphor to describe how he will be taken out of his "city" (i.e. called to his fated death):
It is like the officer who engaged a comic actor dismissing him from the stage. 'But I have not played my five acts, only three.' 'True, but in life three acts can be the whole play.' (12.36)
Marcus concocts this little scene to remind himself that the determination of a full life has nothing at all to do with his readiness for death. Like an actor, he must follow his script and obey the cues when they are given. He is not, in fact, the cosmic playwright, who will clearly retain creative control over all the exits and entries on this world stage. Marcus himself just has to go along with it.