With Reporting by Josh Horowitz

Warning: Major spoilers for "X-Men: Days of Future Past" lie ahead.

"X-Men: Days of Future Past" is without peer in the superhero movie genre, in terms of the sheer size and scope of its cast. Mutants from two different generations collide in a battle-for-it-all against the Sentinels, mutant-hunting machines that ravage our heroes in both the distant past and the not-too-distant future.

But walking out of the theater, fans were surprised to see a trio of unexpected X-Men resurface: Famke Janssen as Jean Grey, James Marsden as Scott "Cyclops" Summers, and Kelsey Grammer as Hank "Beast" McCoy. With Cyclops and Jean dead, and Grammer's Beast not seen since Brett Ratner's "X-Men: The Last Stand," it was surprising to see these characters in the bright-and-happy future created by Wolverine and young Charles Xavier.

We know how these characters returned from beyond the grave in the context of the movie. But how did the characters come back without fans noticing?

In the case of Grammer's Beast, the "Transformers: Age of Extinction" star told MTV that he was sworn to secrecy about his cameo, and was tasked with flying on "the tiniest jet you had ever seen" leaving John F. Kennedy airport in New York to arrive on the Canada set of "Days of Future Past" without drawing attention. Even then, rumors floated around that Grammer was filming a secret "X-Men" cameo.

Logistics aside, what was it that convinced Grammer to return as Beast? Apparently, it didn't take any convincing at all.

"I had the best time with all of that," the actor told us during the "Transformers" junket. "He's one of my favorite characters. It's a guy who hasn't been mined, at least in that manifestation, as well as he could."

But Grammer wanted to make sure that if he was returning to Beast, it was on his terms.

"When we revisited my makeup, I called up Bryan and said, 'I know it's not a big part, but I feel connected to it; it's my role,'" he said. "And he said, 'I'm with you on that.' So we redid some of the makeup."

With Grammer's input in Beast's redesign, the actor was all aboard for the quick scene. And with any luck, it's not his last time in the role.

"I would love to revisit Beast," he said. "You never know."