Robert Downey Jr. says he and the Avengers: Doomsday cast "nerded out" over the original X-Men stars

Marvel Studios’ Avengers: Doomsday continues to generate excitement ahead of its December 2026 release, and Robert Downey Jr. has now shared what it was like working alongside the original X-Men cast. During a post-Hall H interview at San Diego Comic-Con, the actor revealed that seeing Rebecca Romijn and Kelsey Grammer reprise their iconic mutant roles created an unforgettable atmosphere on set.

The comments come as Marvel prepares one of its biggest crossover films yet, bringing together Avengers, Fantastic Four, Thunderbolts, and several legacy X-Men characters under the direction of the Russo Brothers.

Robert Downey Jr. says returning X-Men stars energized the cast

Speaking with Entertainment Weekly, Downey Jr., who portrays Doctor Doom in the upcoming film, said the cast couldn’t hide their excitement after Romijn and Grammer returned as Mystique and Beast.

“It was a big moment, you and Kelsey being back,”

Downey Jr. told the pair before adding,

“I have to say, we nerded out.”

Although Marvel continues to keep the film’s story tightly under wraps, Downey Jr. explained that the pair’s return had an immediate impact on the production.

“Their presence on set lit up this constellation of excitement. It’s just incredible,”

he said.

Romijn and Grammer originally debuted as Mystique and Beast in Fox’s X-Men films before Marvel Studios began integrating legacy characters into the Marvel Cinematic Universe following Disney’s acquisition of 21st Century Fox. Their return also reunites them with fellow original franchise stars Patrick Stewart, Ian McKellen, James Marsden, and Alan Cumming, all of whom are confirmed to appear in Avengers: Doomsday.

Kelsey Grammer reflects on returning as Beast

Kelsey Grammer returns as Beast, bringing the fan-favorite mutant back to the Marvel Cinematic Universe. (Image via 20th Century Fox, Marvel Studios)

Grammer said stepping back into the role after two decades felt natural because of his admiration for the character.

“I love the character. I have this sense of this kind of nobility about him that he carries himself with, a deep respect for all life,”

he told Entertainment Weekly.

The actor also spoke about why Beast continues to resonate with audiences.

“I think that’s what we’re all here for… the hope that actually there is goodness in all of us and that good still triumphs,”

Grammer said.

Beast last appeared briefly in the MCU during the post-credits scene of The Marvels, making Avengers: Doomsday his first major Marvel Studios appearance.

Rebecca Romijn says Mystique’s transformation has become much easier

Romijn also discussed returning as Mystique, revealing that modern filmmaking has significantly reduced the lengthy makeup process she endured while filming Fox’s original trilogy.

“Apparently, they were no longer interested in waiting for actors to be processed, so it was much faster,”

she joked.

Despite the improvements, Romijn admitted the original transformation process helped shape her performance.

“The original transformation that I went through for those first three X-Men movies was really imperative to that character for me. It really put me in just the right headspace to play a character as angry as Mystique,”

she explained.

Avengers: Doomsday is scheduled to arrive in theaters on December 18, 2026, marking one of Marvel Studios’ most ambitious crossover events as it unites heroes from across multiple generations of comic book films.