When Prince Charles ascends the throne after Queen Elizabeth II’s death, he will have to fill the sensible pumps of the longest-reigning monarch in British history. How that might play out is the subject of “King Charles III,” Mike Bartlett’s play opening at the Shakespeare Theatre Company on Tuesday.

In the speculative drama, Charles must learn how to preside in a modern kingdom where many of the people have known only one queen — and where his son is more popular than he is. The play, largely written in blank verse, echoes many of Shakespeare’s royal plays, with themes of power and family tensions, plus a ghostly cameo.

Robert Joy plays the current Prince of Wales, a role that required great balance. “My early career was in sketch comedy, and there’s an attraction to the lampooning of the character,” he says. “But you realize with a play like this, that’s not enough. That’s not the goal at all, in fact.”

His job as an actor was to dig into the imagined private life of Prince Charles, an investigation that started with understanding Charles’ public persona. “We’ve all studied the characters we’re playing via YouTube videos and reading books and articles, and you do pick up mannerisms and sounds and impressions of the physicality of the people,” Joy says. “The responsibility of representing that person honestly and faithfully and respectfully is paramount.”

And that’s easy, at least when he’s playing the face the new king shows to the public. Then comes the part YouTube can’t help with, particularly since both the plot and the characters of “King Charles III” are entirely imagined. While Joy is playing a real person, it’s a person whose true reality is almost always hidden. “You realize the limitations of the research because everything that we can see, everything that we can research as actors, are moments where these people are in front of the cameras, there are bright lights shining on them, there’s microphones in their faces.”

He learned you can’t even trust the candid scenes of the royals. “There’s lots of footage of [Charles] — let’s put it in quotation marks — ‘hanging out’ and playing games and him as a father with his young kids playing on the lawn, but you realize how it’s all scripted and massaged. It’s hard to get at what might be behind that, and it’s folly to believe that these people might not have the same range of emotions that all the rest of us have.”

The play’s audience sees that tug between public and private in the very first scene, which begins with a royal procession. “And then immediately afterwards, you see Charles recoil at how he’s had to present himself,” Joy says. “You say, ‘Here’s this Faberge egg, here’s what we’re about.’ And then right after that you crack the egg open and not only is it a front, but inside there is a very messy yolk.”

Sidney Harman Hall, 610 F St. NW; Tue.-March 12, $44-$118.

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