The fifth episode of WandaVision has provided possibly the biggest upheaval to the MCU ever, with the arrival of Evan Peters as Wanda's brother Pietro aka Quicksilver from an alternate reality. His arrival seemingly confirms the existence of the multiverse and the fact that Fox's X-Men films are now a part of the MCU. In a recent interview, Kat Dennings, who plays Darcy Lewis on the show, explained just how hard Marvel worked to keep Peters' appearance a secret.

"Obviously, that was a massive surprise. I mean they kept it from me a little bit as well. I wasn't sure what was going on. While we were shooting, they cloaked him in [this] thing, so no one could take a drone shot of him getting out of a van, or whatever it was. It was a big secret, but, They pulled it off."
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The arrival of the new Quicksilver came just as Wanda and Vision were about to come to blows over the dark truth of the sitcom reality that they exist in. Peters' character came in just in time to add another bewildering puzzle to the tragedy of the town of Westview that the series takes place in. Pietro also fulfilled a narrative purpose within the "sitcommy" nature of the world that Wanda has created. WandaVision showrunner Jac Schaeffer had previously explained that the addition of the new Quicksilver is meant to be a riff on the "kooky relative who drops by for a short visit" trope.

"We thought like, how do we give him this entrance, and then enjoy that, and then make it crazy? And we had long had the idea of the trope of the brother, or the relative, or whoever comes to town and like, stirs things up with the family - that sitcom trope... We were rooting for it for so long, and didn't know if it would be possible. It was complicated to make happen."
"Evan was always up for it - like, always, always, always. He is a comic book fan, and a Marvel fan. He is always up for the absolute weirdest option. And he's a pleasure - truly a pleasure to work with. Everybody was really excited. I think Kevin [Feige, Marvel Studios president] wanted to make sure that there was a reason for it, that it made sense. And I hope that's what we did."

The sixth episode of the show has hinted that all is not well with this new Quicksilver. Only future episodes will tell whether this new Pietro is going to be sticking around, or Wanda will have to lose her brother all over again.

Written by Jac Schaeffer and directed by Matt Shakman, WandaVision stars Elizabeth Olsen as Wanda Maximoff/Scarlet Witch, Paul Bettany as Vision, Randall Park as Agent Jimmy Woo, Kat Dennings as Darcy Lewis, Teyonah Parris as Monica Rambeau, and Kathryn Hahn as Agnes. The show takes place in an artificial reality that Wanda has constructed for herself and her android boyfriend Vision modeled on classic sitcoms. New episodes air Fridays on Disney+.