Evan Peters joined "WandaVision" as a completely different version of Quicksilver, but there's a reason behind his appearance.

In one of the scenes of "WandaVision" Episode 5 titled "On A Very Special Episode," Scarlet Witch (Elizabeth Olsen) and Vision (Paul Bettany) were seen fighting over the sitcom reality Wanda created.

When the doorbell rang, Wanda then assured her partner that she was not the one who made it.

Upon opening the door, she stood there in shock as she saw her brother, Pietro a.k.a Quicksilver.

It is worth noting that the character, played by Aaron Taylor-Johnson, died on "Avengers: Age of Ultron." In the same 2015 blockbuster team-up movie, MCU fans scored a glimpse of Wanda for the first time.

However, in the new series, Evan Peters played the role, introducing a different version of the character that only existed in another cinematic franchise.

While this stirred questions among the viewers, its head writer Jac Schaeffer also could not believe that Peters' appearance really happened.

Why Evan Peters' Quicksilver Is on MCU?

Schaeffer told marvel.com that they have always loved the idea of bringing him back. Peters' character last appeared on "X-Men: Apocalypse" in 2016

"Like, how do we justify this? Because that's the thing, you can hatch a million great ideas, but to make them land, to make them be grounded, to make them feel organic to the larger story," she went on.

Schaeffer and executive producer Mary Livanos reportedly wanted the crossover to happen since day one.

Per the head writer, "WandaVision" is definitely one of the most mind-boggling series Marvel Studios created for Disney Plus.

Since the series shows what's real and what's not at the same time, they decided to add more thrill by bringing Peters' Quicksilver to the Marvel Cinematic Universe.

Elsewhere in the interview, she served the "crazy" idea they came up with.

"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," she went on.

Fortunately, Peters himself approved the plan and helped the creators make it happen.

Being a comic book and Marvel fan himself, the 34-year-old actor joined the fun and came up with the weirdest option.

Meanwhile, Schaeffer assured that Marvel Studios President Kevin Feige approved the turn of events after realizing the reason why they wanted to adapt the plot.

READ MORE: WandaVision Spoilers: 3 Most Shocking Revelations in Episode 5