Director James Gunn has confirmed Nathan Fillion's reported cameo as Simon Williams/Wonder Man in Guardians of the Galaxy Vol. 2 - though he has also confirmed Fillion's cameo was cut from the film.
In a Facebook post, Gunn explained that he gave his personal friend Fillion the role of Simon Williams, a superhero/actor, because he thought Fillion would fit the part, and because he wanted to give Fillion a cameo that could become a full-fledged role in later films. Fillion's cameo was limited to a series of Simon Williams movie posters seen on a theater wall in a deleted scene from Guardians 2.
Fortunately, according to Gunn, Fillian's cutting-room-floor-cameo can still be considered canon for the MCU. Here's Gunn's statement in full:
"As many of you know one of my best friends in the world is Nathan Fillion. Along with Michael Rooker and Gregg Henry he's been in every movie I've directed - even as a monstrous, perverted voice in Guardians of the Galaxy. I didn't have a good cameo for him in Vol. 2 - and I wanted to bring him more fully into the MCU at some point, so I didn't want to make him Arkon Guard #2, narrowing his chance of a more substantial role in the future. I really do love the character of Simon Williams/Wonder Man in the comics - a sometimes douchey actor/superhero - and could see Nathan clearly in that role (not because he's a douchebag but because he's great at playing one). But, of course, in a movie set 99.9% in space I didn't really have a place for him. So in a small flash to earth I decided to put a theater playing a "Simon Williams Film Festival," with six Simon Williams movie posters outside. Obviously, from the posters, he's had a run of B movies. Most of them in themselves are Easter eggs of some sort or another. Unfortunately, the small section of the scene where they appeared slowed down the movie and I had to cut the Easter eggs from the film (along with storefronts named after comic book luminaries Starlin, Mantlos, Abnett and others). Equally a bummer was that a lot of people took photos of these posters on the day so suddenly every fan site was reporting that Nathan was playing Wonder Man in he movie. He was even the third-billed actor on IMDB!
"So that's the full story. Nathan's only cameo in the movie ever were these posters. I'll post them all over the next few days.
"And, yes, I think we can consider them canon for the MCU, and I hold onto hope that Simon Williams will rise again!!"
Fillian has appeared in nearly all of James Gunn's films, and even had a cameo voicing a large blue alien inmate in the first Guardians of the Galaxy film. Along with the statement, Gunn revealed one of the fake movie posters made for the movie, seen here.
In comic books, Simon Williams was briefly a member of the original Guardians of the Galaxy team under the name "Hollywood." Some characters from that iteration of the team appeared in Guardians of the Galaxy Vol. 2.