LONDON – They are a merry band of mutants, at least when the director is away and the hard work is done.

They've been given a task — concoct a "prequel" that will satisfy longtime fans of the "X-Men" series and bring in new moviegoers as well — and, with global release just a few days away, they think they've nailed it.

Much of the cast gathered in London recently to boast about the film — tastefully of course — at a round-table discussion that focused on the challenge of creating a credible early life for comic strip characters already portrayed successfully in four films by such masters as Patrick Stewart and Ian McKellen, venerable English actors who carry the title "Sir" in front of their names.

This time, it's a much younger cast playing the mutants in their formative years, when they were still discovering and honing the special powers that set them apart from what they view as the rather drab human race. As a result, "X-Men: First Class" is filled with soul-searching identity crises as the mutants wrestle with a central dilemma: To downplay their differences in order to be accepted by humanity, or to celebrate what makes them unique, humanity be damned.

Instead of McKellen and Stewart in the key mutant roles of Magneto and Professor X, it's Michael Fassbender and James McAvoy, starting off as allies but ending up as bitter foes. The closest thing the cast has to eminence is Hollywood veteran Kevin Bacon, who plays evil mutant Sebastian Shaw with villainous glee.

Fassbender, a talented actor of German and Irish descent, said he did not feel hemmed in by earlier portrayals of Magneto, even if his approach doesn't appeal to fans of the earlier movies, which turned the old Marvel comic into a lucrative international film franchise that started with "X-Men" in 2000.

"I think we all realize there's a massive fan base out there and we definitely want them to like it," said Fassbender, seen in 2009's "Inglourious Basterds." "They are the first sort of go-to audience, but there has to be a certain amount of disrespect for them as well, because you're trying to do something new. You're trying to make decisions that you think are justifiable and you have to forget about that or you can end up not making any bold choices. And I think we all made bold choices and took risks."

McAvoy, his voice still carrying a heavy hint of his native Scotland, said that means the new cast is to blame if the movie bombs — a fate that would sink plans for two additional "X-Men" prequels and a chance for the franchise to continue a few more years at least.

"It is intimidating because the four films made a lot of money, so clearly people like the characters enough to go and see them," said McAvoy, who starred in "The Last King of Scotland" and "Atonement." "If it doesn't work, we take full blame."

He said his approach to Professor X was to show how different the character was as a very young man just discovering the range of his phenomenal telepathic powers. Director Matthew Vaughn had made it clear at the start of filming that he did not want McAvoy and Fassbender to simply portray younger versions of Stewart and McKellen.

Vaughn's approach meant developing an inner life and a back story for the characters, and playing them in the turmoil of youth, when their personalities are still being forged.

Fine, but isn't it a bit absurd working out a complex inner life for comic strip characters? A case of overkill in the motivation department?

No way, said Bacon, who handled Sebastian Shaw's sociopathic tendencies with care.

"You can never have too much back story," he said. "For me at least, if there's no back story in the movie then you look for some kind of source material, and if there's no source material, you make it up. You sit there and you write it: 'I was born in this town and this is what my daddy did, and here's my playlist of songs I like to listen to.' For me, that's what it's gotta be."

The film takes place in the 1960s — the height of the original Marvel comics era — and gives Bacon's character a key role in a highly fictionalized version of the 1962 Cuban missile crisis. The plot device gives the director a chance to use actual footage of President Kennedy and Soviet hothead Nikita Krushchev, remembered for banging his shoe on a table during a spirited United Nations debate.

The '60s setting is exploited by the set and costume designers — the cleavage-boosting outfits worn by January Jones as Emma Frost are the most obvious examples — but they also provide a wistful quality to the mutants as they search for themselves.

"A lot of the characters are more innocent," said McAvoy. "Certainly my character is much more innocent, he's not tainted."

The youthful rebellion of that era is mirrored to some degree by the mutants, who can't decide whether to trust or obliterate the humans who seek their help.

Fassbender said the fans identify with the mutants' struggle for identity and respect. The new film shows how the young mutants find one another — and bond out of deep relief that they are not alone.

"It gives them hope to find other people are experiencing the same thing as they are," he said. "You know, it's a horrible feeling to think, oh my God, I'm on my own. I'm going through this by myself. But no, there are actually other people going through the same thing."

He said the genetic mutations are "the handicap that can actually become a special quality."

McAvoy's take is that the mutants all have terrible lives, full of angst and rage, but also find they are terribly special because of their secret abilities.

"That's the thing about every mutant, isn't it?" he said.