The subsequent franchise films highlighted the complex bond between the characters, ideologically at odds over how to protect mutantkind.

On screen they sparred, but off set they bonded.

“We have to thank ‘X-Men’ for the two of us really getting to know one another, because every day on an ‘X-Men’ movie you spend far more time sitting in your trailer than you do on the set doing a drop of work — and those trailers are extremely luxurious. And we had adjoining ones, so we sat around and talked and reminisced,” Stewart said on the 2014 panel. “And I discovered a lot about this chap and he about me, and we found that there were many, many overlaps in our careers and many areas where we had little in common.”

The rest is history. The two shared a friendship adored by fans, often seen greeting each other with an affectionate kiss on the lips whether on TV programs or on the red carpet.

Stewart told The Guardian that McKellen is “very close” to his wife, singer-songwriter Sunny Ozell, and that she was the one who suggested McKellen officiate their wedding. In 2013, he did just that.

Stewart was also a supporter of the LGBTQ+ community and praised activists like McKellen, who is gay.

“I think this is where the theater is such an appealing world, because it embraces everything and always has. So there was never a moment where I made an intellectual choice that I would be a supporter of gay civil rights. It was always a natural and uncomplicated choice,” he told The Advocate in 2015. “Then later in life, as I got to know well-known activists like Ian McKellen, I was only too happy to join campaigns, march, and support in whatever way I could because it always seemed to me to be something that much too much fuss was being made about.”

Both men built separate and successful legacies in cinema — McKellen as the beloved wizard Gandalf in the “Lord of the Rings” and “The Hobbit” films and Stewart as Capt. Jean-Luc Picard of the long-running “Star Trek” franchise.

They were also both honored with knighthood by Queen Elizabeth II — McKellen in 1991 and Stewart in 2010.

Through it all, they remained devoted to their first love of theater and co-starred in two productions, to much acclaim.

In Samuel Beckett’s play “Waiting for Godot,” Stewart was the optimistic Vladimir and McKellen the weary Estragon. Their performances — at London’s West End in 2009 and on New York’s Broadway in 2013-14 — were praised for their humor and comedy.