Evan Peters on How 'Shocking' 'Deadpool 2' Cameo Took the 'X-Men' Cast by Surprise (Exclusive)

The 'X-Men: Apocalypse' star also opened up to ET about working with his fiancée, Emma Roberts, and his 'intense' upcoming crime drama 'American Animals.'

Since exploding into theaters earlier this month, Deadpool 2 has been wowing audiences with its brash humor, unbridled action sequences and its slew of big-name blink-and-you'll-miss-it cameos.

Among the many stars who unexpectedly showed up in the big-budget sequel was Evan Peters, reprising his role as the X-Man Quicksilver from X-Men: Apocalypse

The 31-year-old actor appeared alongside his X-Men co-stars Nicholas Hoult, as Beast, Alexandra Shipp, as Storm, Tye Sheridan, as Cyclops, Kodi Smit-McPhee, as Nightcrawler, and even James McAvoy, reprising his role as Charles Xavier.

Peters recently sat down with ET's Deidre Behar during a press junket for his upcoming crime drama, American Animals, and he opened up about how the epic cameo came to be.

"We were shooting [the upcoming X-Men: Dark Phoenix], and they were like, 'Hey we're going to do this quick thing for Deadpool 2 real fast, we gotta get everyone together so let's do it," Peters recalled.

In the film, Deadpool (Ryan Reynolds) finds himself inside the X-Mansion. In a call-back to the first Deadpool film -- when they didn't have the budget for an appearance from any of the canon X-Men aside from Colossus -- the Merc with a Mouth goes on a fourth-wall-breaking rant asking why the mansion was still empty, despite the movie having more money this time around.

While distracted by his own profane diatribe, Deadpool fails to notice all the of the X-Men standing together in a room behind him, and they quickly close the door to avoid being seen by the loud-mouthed anti-hero.

It was one of the funniest and most self-referential moments in the film, and Peters said that they actually filmed several takes with their characters doing something different each time.

"We did a bunch of different versions, some not PG, and they ended up using that one," Peters said. "I thought it was going to be for the end credits -- and I think everybody kinda did -- but it ended up being like, boom, right there sort of in the first half of the film. Which was really sort of weird and shocking."

The X-Men stars' brilliant cameo is just one of several throughout the film -- including two appearances from some real A-listers (and former Ocean's Eleven co-stars), that are just too good to spoil.

As for his role as Quicksilver, Peters has now played the supersonic speedster in three X-Men films -- Days of Future Past (2014), Apocalypse (2016), and the upcoming Dark Phoenix, scheduled to hit theaters in February 2019. 

However, Peters told ET that he "would love" to star in a Quicksilver standalone project if the opportunity ever presented itself.

"I think that would be so fun," Peters said. "I love the special effects and the sequences and I love working with all those guys, so in one second I would do that."

Another filming opportunity that Peters would love to have would be to shoot a movie with his fiancée, and former American Horror Story co-star, Emma Roberts -- preferably in some tropical locale.

"I would love to go do a movie with her somewhere, that would be incredible!" Peters exclaimed. "[Somewhere like] Bali. Let's go to Bali!"

He also reflected on what it's like getting to work on set opposite Roberts, who he's been dating since 2012, and engaged to since 2014.

"We're together all the time and we're best friends so I think that everything just becomes that much easier and all the stuff that sort of can upset you or annoy you or frustrate you, we kind of look at each other and then just laugh about it and get through it," Peters shared. "I just think it's really fun working with her, I have a great time working with her."

However, before he gets to share the screen with his lady love in any future projects, Peters' upcoming crime drama American Animals will be hitting the screens.

According to the young star, his role in the thriller -- which centers around an audacious and ill-fated heist -- was challenging, especially when it came time to filming the heist itself.

"I was exhausted and completely stressed out," Peters said. "It was intense, it was definitely an intense sequence to shoot but it was fun."

The role also proved hard for Peters to shake off when he wasn't on-set, although the actor admitted that it was also a choice on his part to give more authenticity to his performance.

"I hardly went out, I hardly saw anybody. Once we started shooting I was sort of this recluse," he said. "I wanted to stay focused, I wanted to keep my blinders on, I wanted to be in the movie and I wanted to really try to accomplish that and get it right. So in a way, it was kinda with me the whole time."

American Animals hits theaters June 1. Check out the video below for a look at the intense crime caper.