Aubrey Plaza thinks Marvel fans are going to go wild at her character reveal, and everyone is making the same joke

Aubrey Plaza is making her MCU debut in Agatha All Along, but so far her character Rio Vidal is shrouded in mystery. 

Naturally, that promotes fan theories – and Plaza herself has fanned the speculation flames by addressing the enigma that is her character. "I'm just so happy they let me play this character," she told Scarlet Witch Updates. "I can't spoil it, but what happens, the reveal is pretty amazing. And I think for the fans especially, they're going to go crazy."

WandaVision spurred tons of theories back when it was releasing on Disney Plus, so it's no surprise that Plaza's comments have spurred the re-emergence of an old joke. "I'm calling it: Mephisto," says one person: the villain was heavily speculated to appear in WandaVision, but he never showed up, despite all the fan theories. 

"Aubrey Plaza as… Mephisto," jokes another fan. Another person echoes: "Holy shit guys, Aubrey Plaza is going to play Mephisto"

Of course, some other, more serious theories have also sprung up. "Rio Vidal means river of life which refers to the first green witch Gaia AKA Mother Earth. Aubrey Plaza is most likely playing her and not Mephisto in the MCU," speculates one person. 

"Aubrey Plaza is playing Cynthia Von Doom in Agatha All Along bookmark this tweet," guesses another fan. "It's just a hunch, nothing confirmed but I think it makes a lot of sense considering Avengers Doomsday coming up and how his mother is vital to his story. She was a witch and has ties with Mephisto who is also rumored to appear in the show"

"Morgan le Fay obviously. She even looks like her," says someone else. There's not long to wait to find out if any of these theories are true, either, with the first two episodes of Agatha All Along debuting on Disney Plus this September 18. 

While you wait, check out our guide to all the upcoming Marvel movies and shows for everything else the MCU has in store, or plan a marathon with our explainer on how to watch the Marvel movies in order. If you're short on time, though, then try the Marvel movies and shows to watch before Agatha All Along.