Several mutants from the first X-Men trilogy and 2011’s First Class are slated to appear in the upcoming Days of Future Past. But at least one cameo has managed to fly under the radar until now. According to Showbiz Spy, Kelsey Grammer, who appeared as Beast in 2006’s The Last Stand, will appear in the film.

“Producers had the brilliant idea to bring back Kelsey for Days of Future Past,” said an unnamed source at the site. “It wasn’t hard to get him to sign on for a guest appearance. He jumped at the opportunity the second it was brought to his attention. Kelsey’s special cameo was secretly shot during post-production on the Fox lot under the impression that Kelsey would be on set to film a pilot for a different project.”

It’s unclear how Beast will impact the story, if at all.

In Days of Future Past, the younger Beast, played by Nicholas Hoult, transforms in and out of his animal form. In a recent interview, Hoult said, “What’s happened up to this point is [that] between the time of the last movie and this movie, my character has created a serum which basically controls his mutation, so his appearance is normal as long as he doesn’t get worked up.”

The second full-length trailer for the movie will arrive this week. Meantime, check out the latest teaser trailer below. X-Men: Days of Future Past comes to theaters on May 23, 2014.