Rosencrantz and Guildenstern Are Dead is a play by Tom Stoppard that premiered at the Edinburgh Festival Fringe in 1966. Its protagonists are two minor characters from Shakespeare's Hamlet, the courtiers Rosencrantz and Guildenstern. This choice of protagonists seems to reflect the seminal line from Eliot's "The Love Song of J. Alfred Prufrock":
No! I am not Prince Hamlet, nor was meant to be;
Am an attendant lord, one that will do
To swell a progress, start a scene or two ...
In other words, these are supporting characters whose very existence is simply intended to fill in the stories of other people. Stoppard, by making them protagonists, raises a question of existence and metatheater.
In terms of metatheater, making an audience aware explicitly of the nature of minor characters makes viewers question the nature of theater itself. Characters are artificial constructs whose theatricality and unreality is highlighted by the lack of continuity in their narratives. Unlike real people, these characters seem to have no story or existence until they are paraded onto stage to support the narrative of the protagonist. The emphasis on what happens behind the scenes and between their appearances in the plot does what is called "breaking the fourth wall" or constantly reminded the audience of the performance's nature as a performance.
On a deeper level, this makes the audience think about the nature of people in reality. We only encounter others in brief intervals. For example, we might have relatives we see only on holidays or friends we only encounter occasionally in real life or on social media. In a sense, we are all protagonists of our own lives and other people are supporting characters within them. This play reminds us that what we might see as "supporting charterers" have fully realized lives outside our own and that our own realities are only "real" to them when we appear on their mental stages.