With the casts of the Arrowverse's various shows coming together for "Crisis on Infinite Earths," the upcoming crossover event still has a number of surprise cameo appearances that haven't been announced or leaked.

The crossover's showrunner and executive producer Marc Guggenheim revealed there at least six additional cameos within the first three installments of the crossover have not yet been revealed, with even more planned for the event's last two installments.

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"Yeah, I like how there are at least six characters who appear in the first three hours that we haven’t announced. And I’m working on even more for the last two hours," teased Guggenheim in an interview with IndieWire. "So we’ve obviously announced a bunch, but I really want the audience to be able to tune in and still be surprised. So there are surprises of sorts that we’ve managed to keep secret, and not without great difficulty, I will tell you."

With plenty of major cameos -- including Kevin Conroy appearing as Kingdom Come Batman and the Smallville Superman and Lois Lane returning -- one has to wonder who are the characters that have yet to be included in the coming Crisis.

"Crisis on Infinite Earths" begins Sunday, Dec. 8 at 8 p.m. ET/PT on Supergirl, then continues in Batwoman on Monday, Dec. 9 at 8 p.m. ET/PT and in The Flash on Tuesday, Dec. 10 at 8 p.m. ET/PT. After the winter hiatus, the crossover will conclude on Tuesday, Jan. 14 in Arrow at 8 p.m. ET/PT and in DC's Legends of Tomorrow at 9 p.m. ET/PT.

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