He may be a high-ranking officer of S.H.I.E.L.D., but Samuel L. Jackson isn't very good at keeping insider secrets.

The man known to Marvel fans as Nick Fury unilaterally gave Wall Street Journal readers a top-secret security clearance (probably much to Marvel's dismay) when he let slip that Elizabeth Olsen has joined the cast of one of the most anticipated sequels of all time, "Marvel's The Avengers: The Age of Ultron."

Jackson, who made a cameo appearance as Fury in this week's episode of NBC's "Agents of S.H.I.E.L.D.," mentioned Olsen's involvement when asked about the status of Joss Whedon's highly guarded comic book epic.

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"I know we're shooting in London, that James Spader is Ultron and going to be the bad guy, and that we added Ms. [Elizabeth] Olsen, but I don’t know what she's doing, if she's on the inside or the outside," said Jackson. "I haven’t seen a script."

Olsen, the younger sister of legendary child stars Mary-Kate and Ashley Olsen, burst onto the scene with her harrowing performance in 2011's "Martha Marcy May Marlene." The fanboy rumor mill has been churning in recent months with talk that the younger Olsen had been slated for a role in the "Avengers" sequel. Many have speculated that she'll be playing the Scarlet Witch, one half of the "brother-sister act" that Whedon first mentioned at the premiere of "Iron Man 3."

Twin siblings Pietro Maximoff, aka Quicksilver, and Wanda Maximoff, aka the Scarlet Witch, made their comic book debut in "X-Men" No. 4 in 1964. As the offspring of Magneto, they were portrayed as bad guys for a few issues before switching sides and joining up with the Avengers in 1965. Scarlet Witch is gifted with the power to manipulate probability and reality via her "hexes," which usually manifest as cool-looking "hex bolts" or "hex spheres," while Quicksilver is able to move and think at super-sonic speeds.

This isn't the first time that Samuel L. Jackson has let a Marvel-related scoop out of the bag. During the press conference for "Captain America: The Winter Soldier" at this summer's San Diego Comic-Con, Jackson mentioned that the upcoming political thriller will feature an interesting new character arc for Black Widow (Scarlett Johansson).

[Related: Mutant Twins Mania! Quicksilver and Scarlet Witch Confirmed For 'Marvel's The Avengers 2']

"She might have something of a 'whistleblower' in her future," said Jackson, to which the Captain himself, Chris Evans, responded with a playfully incredulous "SAM!" and told him to shush.

Before we supposedly see them together in "Avengers 2," Jackson and Olsen will appear together in Spike Lee's "Oldboy," which opens on November 27. "Age of Ultron" will begin production next spring with a likely release date of May 1, 2015.