Ben Affleck on Why He Wanted to Play Batman in 'Batman v Superman'

— -- Ben Affleck may be the new man in the Bat-suit in “Batman v Superman: Dawn of Justice,” with major Bat boots to fill, but the actor says he’s long been drawn to the depths of the Dark Knight.

“Batman's the most interesting superhero in a way because he's the most human, you know. He's the most like us,” Affleck told ABC News' “Nightline.” “And he can be kind of broken, which is really fascinating to be coupled with these -- all this heroic stuff.”

The story pits the two mightiest icons of the DC Comics universe against each other, and its stars know it has one of those movie titles that makes fans go, “wait, what?”

“It's counter-intuitive,” Affleck said of the title. “Because you think, A, well, they're both good guys, and, B, how could Batman really fight Superman since Superman is an alien and way stronger than him, and invulnerable?”

Affleck takes on 32-year-old Henry Cavill’s Superman/Clark Kent, and at 43, Affleck is the oldest actor ever to be cast as Batman/Bruce Wayne.

Director Zack Snyder said he pursued Affleck for the Batman role because he had “wanted a Batman that had been Batman for 20 years.”

“He had the experience that Batman that we know sort of collectively in pop culture,” Snyder said. “And I felt like that character had a chance against Superman because he could outthink him.”

“I wanted to be in one of these movies that was ballsy, and that had something to say,” Affleck added. “And I wanted to do a movie that my kids would think was cool. ... My son thinks it's cool, at least.”

Samuel, the youngest of Affleck’s kids with Jennifer Garner, was the reason Affleck ran into Christian Bale -- the actor who excelled at Batman/Bruce Wayne for director Christopher Nolan in three Batman epics, grossing more than $1 billion domestically and and helped elevate the superhero movie genre.

“I was getting my kids Halloween costumes,” Affleck said. “And I was literally in a costume shop. And not only that, but my son, who is really into Batman, wanted to get a Batman costume. So I was in the Batman aisle. And it's 11:00 in the afternoon, you know, some random Tuesday. No one's there. And I hear this voice say, ‘Hey. Ben, is that you?’”

And when he turned around, Affleck said there was Bale.

“He's incredibly sweet, really cool. And he's talking to me about going off, and doing the movie,” Affleck continued. “And he's like, ‘Listen, make sure they put a zipper in that suit. I couldn't take a piss for three movies.’ So, sound advice. And it was heeded.”

Inside the suit, Affleck worked hard to get a physique painstakingly primed to superhero standards -- an absolute necessity given Cavill’s Instagram-mically impeccable pecs.

“That was very daunting,” Affleck said of Cavill, laughing. “I thought, ‘If I have to be as in good a shape as this guy, I think I'm in trouble.’”

Affleck said he worked out for almost a year before “Batman v Superman” started filming to become a superhero at his “advanced age.”

“And that does not come easy,” Affleck said. “But that's what audiences have come to expect. ... I look at the Adam West Batman now, and I think, ‘God. They just -- guy's got away with murder.’”

“Batman v Superman” uses multiple story strands -- and Jesse Eisenberg’s Lex Luthor, the string-pulling villain -- to put these two do-gooders at odds. The grim obsessions of Bruce Wayne, billionaire vigilante, are set off against the incredibly scrutinized alien Superman, who is wide-eyed and in love with Lois Lane, played by Amy McAdams.

“If you were to just have head-to-head Batman against Superman punch-up, we all know who wins,” Cavill said, laughing.

The film’s full title, “Batman v Superman: Dawn of Justice,” cues fanboys and girls that Snyder is also looking to amass the Justice League, DC’s all-star array, for future “Avengers”-style films, with a butt-kicking super-sylph known as Wonder Woman, played by Gal Gadot.

“My most fun day on the set was when my kids came to the set, and saw us in the costumes,” Affleck said. “My son was like, ‘Is that the real Wonder Woman?’ I was like, ‘Yes. That actually is the real Wonder Woman.’”

For Cavill, having children see him in his Superman costume gives him a bit of a different reaction.

“That is probably the scariest part of this,” he said. “Because when a child ... sees you as Superman, there is a lot of responsibility there not to mess up. ... And you're often not expecting it. It's the last minute where, you know, a parent just thrusts their child in front of you, and you have to say just the right things, and you never know what a kid's going to ask either because kids are honest.”

Audiences expected to flock to the movie’s March 25 opening will be able to see if Snyder's “Batman v Superman” as a worthy successor to the Nolan films.