Maisie Williams "Reveals" Huge Game of Thrones Spoiler on The Tonight Show

Remember the date in which she stopped by the late-night show to chat with Jimmy Fallon

You know the saying: Loose lips sink ships. Maisie Williams learned that the hard way on The Tonight Show starring Jimmy Fallon.

While discussing Game of Thrones with host Jimmy Fallon, Williams recounted Arya Stark's defining moments, including the beheading of Ned Stark (Sean Bean), tossing her original costume away and more.

"I've decided to just keep a tight lip on everything. HBO has sent out a lot of memos recently about just saying nothing," she said. "So, there's really nothing I could say. They would absolutely kill me."

When pressed for scoop, Williams said the final days were emotional on set, "because when I found out that Arya died in the second episode."

Cue the panic.

Remember what day she appeared on The Tonight Show: April 1, also known as April Fools' Day.

Williams proved her acting chops with the prank, even trembling as she "realizes" her mistake.

For an extended Tonight Show bit, it was pretty good. You could feel how shocked the audience was.

Game of Thrones premieres Sunday, April 14 at 9 p.m. on HBO.