The 21-year-old actress stopped by The Tonight Show Starring Jimmy Fallonon Monday and put all other April Fools' Day pranks to shame. After revealing that she's "decided to keep a tight lip on everything" about the eighth and final season of the HBO drama because the network would "absolutely kill me" if she gave anything away, she breaks her own rule with a little prodding from the host.
"During shooting, like, the final days were really, like, emotional, saying goodbye to all of the cast. Because when I found out that Arya died in, like, the second episode," she said nonchalantly of her GoT character, before quickly covering her mouth in horror.
"What?! Are you kidding me? That's a spoiler!" Fallon exclaimed following Williams' reveal.
Williams seemed positively horrified by her words, asking if the show was live, if her comment can be edited out and how they'd stop the audience from tweeting the news. The whole time the actress was shaking uncontrollably with tears in her eyes, while the audience sat silently stunned.
Fallon assured Williams that all was well and tried to salvage the interview, but she quickly apologized and rushed off the stage in tears. Fallon seemed unsure how to proceed, initially remaining at his desk before deciding to go backstage to check on Williams. When he made it to the curtain, Fallon pulled it back to reveal a smiling Williams as the pair shouted in unison, "April Fools'!"
"You were good. You were so good. Oh my God," Fallon praised Williams, before teasing his audience, telling them, "We got you guys so bad."
Prior to the spoiler fake-out, Williams discussed her character's biggest moments, picking iconic milestones throughout the series that defined Arya.
"In terms of things that have really defined her, I think number one would be Ned's (Sean Bean) beheading in season one. That was what spiraled her whole arc," Williams said of the first season shocker. "And then she was with The Hound (Rory McCann) for a long time. I think there was a line in maybe season three where she was like, 'One day I'm going to stick a knife through your eye and out the back of your skull.' That was, like, a pretty defining moment where you realized how messed up in the head she'd become."
"And in season five I think she threw away her original costume -- and I was in that costume for four years or something -- so she threw that into the water and it was a really emotional moment," she continued. "I guess last season she reunited with Sansa (Sophie Turner) again, so that was a pretty big moment. And then they both united and killed Littlefinger (Aidan Gillen)."
In a recent interview with Rolling Stone, Williams' onscreen sister and real-life best friend, Sophie Turner, joked that the duo was "a nightmare to work with."
"If you’re working with your best friend, you will never get any work done, ever," Turner, 23, admitted. "Anytime we tried to be serious about anything, it’s just the hardest thing in the world. I think they really regretted putting us in scenes together. It was difficult."
When ET's Leanne Aguilera caught up with Turner, she revealed that Williams, along with the rest of the show's cast and crew, is what makes saying goodbye to GoT so hard.
"I think the thing I will miss the most is being in character, in costume, on these amazing sets and being with this crew of people for 12 hours a day and these other actors in their costumes as their characters," she said. "Like, being on the grounds of Winterfell as Sansa Stark -- I never felt more powerful than I have as her in Winterfell, so I’ve just never been happier on a set than Game of Thrones."
The final season of Game of Thrones premieres April 14 on HBO.