'The Batman' Director Explains Barry Keoghan's 'Unseen Arkham Prisoner' Cameo (Exclusive)

The 'Eternals' star is credited as 'Unseen Arkham Prisoner' in 'The Batman' credits -- what's next for the character?

Spoiler alert! Do not read any farther if you haven't yet seen The Batman. Major spoilers ahead for the end of the film and credits scenes.

Director Matt Reeves is aware that The Batman ends with Gotham in much the same place as it began. Despite the titular masked vigilante, played by the newly-cowled Robert Pattinson, taking down both the serial killer known as the Riddler (Paul Dano) and crime lord Carmine Falcone (John Turturro) and saving the city from a massive, disastrous flood, there is no relief in sight for the Batman himself. There is still so much to do.

"That's kind of the way Gotham is. No matter what, Batman's mission will never be done," Reeves told ET.

That much becomes evident in one of the film's final scenes, which takes place in the always ominous Arkham Asylum. We see the Riddler -- locked up after witnessing the failure of his destructive plan -- meeting one of his fellow prisoners, played by Eternals star Barry Keoghan, who has some grand schemes of his own, as well as a distinctive, menacing laugh.

"I can tell you that that the unseen character is the character you think it is," Reeves said of the "Unseen Arkham Prisoner," as Keoghan is listed in the credits. "But he is not yet that character."

For fans of comic adaptations, there has been no shortage of onscreen likenesses when it comes to Batman's No.1 enemy, the Joker. Following Heath Ledger's Oscar-winning turn as the Clown Prince of Crime in Christopher Nolan's The Dark Knight, Jared Leto and Joaquin Phoenix have both played very different adaptations of the villainous tricker, with Phoenix also winning an Oscar for 2019's standalone Joker.

However, it's clear from Reeves' setup that Keoghan is stepping into yet another unique onscreen iteration of the character.

"It's the early days, and the rogues gallery in the comics really creates itself in reaction to this appearance of a masked vigilante who is declared Batman in Gotham," Reeves explained. "These alter egos and what they declare themselves kinda come in response to that, but no one has done it yet."

"The first one is the Batman, and then Selena Kyle's gonna become the Catwoman. And that character... in Arkham, he will become the character that we all know," he added. "What I loved was the idea of leaning into the idea that he was not that yet, and that you could already tell that that was brewing."

"When you see the Riddler and him together, you realize that, while on the one hand, this is a hopeful moment in Gotham, in that the stranglehold, the power that's been held in place, has been broken, you also realize that it's an incredibly dangerous moment in Gotham, because you realize that already plans are afoot for more trouble."

So are there also plans afoot for more films, in which fans might get to witness the evolution of these heroes and villains? "We'll see," Reeves teased.

For Pattinson's part, he told ET that he's more than ready to suit up again, thanks to Reeves' plans for the future of this cinematic universe.

"I was talking to Matt about it, and it's funny because he keeps saying he’s got ideas," the actor told ET of starring in another Batman movie. "There's so much I love. I'm just so proud of the movie and it would be so fun to do it again."

Until then, fans know that the Penguin will live on to see another day, with Colin Farrell set to reprise his villainous role in an HBO Max spinoff series. "It’ll pick up a short time after the last frame of the film,” the actor said. “So, we’ll get to go on a little kind of left turn off to the world of Oz and how he’s beginning to dream of filling a potential power vacuum that may exist.” 

The Batman is in theaters now.