The leading man also discussed the film's mini 'Game of Thrones' reunion.
While at the British GQ Men of the Year Awards on Tuesday night in London, ET spoke with Madden about the project and the decision to prominently feature a member of the LGBTQ community in the story.
"I think it's hugely important, just the cast is so diverse," Madden stated. "I'm a white male actor and not in the majority of this cast. I'm the minority, which is important in terms of diversity in filmmaking. I think Marvel are really at the forefront of that."
Kevin Feige, the president of Marvel Studios, confirmed the inclusion of the openly gay character last week, during an interview with Good Morning America, where he stated: "He's married, he's got a family, and that is just part of who he is." More details on the character have not been released.
Eternals will also notably include a Game of Thrones reunion. Madden and Kit Harington, who played Robb Stark and Jon Snow on the mega-hit HBO show, will be joining forces again beyond the borders of Westeros.
"Thrilled to work with Kit again," the leading man, who won the Hugo Boss Most Stylish Man award at Tuesday's ceremony, shared. "Thrilled to work with Gemma Chan and Angelina Jolie and Salma Hayek and the whole cast, [an] outstanding bunch of actors."
The impressive lineup for the new film was unveiled at Disney's D23 Expo last month in Anaheim, California, where Jolie revealed to ET that the project is so secretive, she didn't know that Madden's Game of Thrones co-star had joined the production until it was announced on stage.
"I'm not so sure what we can say," Jolie said after the film's presentation. "I still think we're seeing each other behind the scenes, like, 'Did you hear...?' I didn't know Kit was in the film! It's a whole thing. I'm thrilled, but I had no idea."
The film centers on a race of immortals who once lived on Earth and had a hand in human civilization. Madden will be playing Ikaris, while Jolie is taking on Thena. Harington will be portraying the Black Knight.
The film also stars Kumail Nanjiani, Ma Dong-Seok, Brian Tyree Henry and Barry Keoghan.
Eternals will arrive in theaters on Nov. 6, 2020.