Nathan Fillion is most recognized for his role in the short-lived but much-loved Joss Whedon series "Firefly" and for his leading turn in ABC's "Castle." But what some fans may not know is that he also has a pretty extensive superhero background. Check his IMDb page and you'll see that he's lent his voice to a bunch of DC Comics properties, such as the animated "Justice League Unlimited" series and a handful of Green Lantern projects. He even voiced an alien in the first "Guardians of the Galaxy." Now, it appears as if he's being promoted to live-action.

Fillion will be returning to the Marvel Cinematic Universe in "Guardians of the Galaxy Vol. 2," though his role isn't exactly what you would expect.

Set photos emerged that show Fillion's face on several fake movie posters, including one for a Tony Stark film intended to mimic "Steve Jobs." Do a little digging and you may find out that the posters list his leading man as Simon Williams, also known as Wonder Man, a member of various Avengers teams from time to time in the comics.

Williams began his journey in the comics as one of Tony Stark's competitors and as a Wall Street-type crook. He soon joined forces with Baron Zemo (the bad guy Daniel Bruhl is playing in the upcoming "Captain America: Civil War") and received superpowers to fight the Avengers. However, Williams had a change of heart and soon went legit. He then leveraged his superhero fame to become a movie star, which is actually kind of hilarious.

The fake movie posters do not make it clear if Williams is just an actor or a superhero-turned actor. It's possible that Fillion himself doesn't even appear in "Guardians of the Galaxy Vol. 2," and can only be seen in background posters and videos. But we know writer/director James Gunn isn't returning to Earth in the highly-anticipated sequel, so we're not entirely sure how Fillion fits in. But we're intrigued nonetheless.

"Guardians of the Galaxy Vol. 2" will hit theaters May 5, 2017.

Follow Brandon Katz at @Great_Katzby.