Transcript for Ben Affleck on Why He Wanted to Be Batman in 'Batman v Superman'

They are the good guys of the comic book world. For the first time ever, Clark Kent and Bruce Wayne are suiting up for the big screen together, facing on of in an epic battle that might become an epic partnership. Chris Connelly goes behind the scenes with Ben Affleck and Henry Cavill in "Batman vs. Superman: Dawn the justice." Reporter: He may be the new man in the bat suit. With major bat boots to fill. Ben Affleck says he's long been drawn to the departments of the dark knight. Batman's the most interesting superhero in a way because he's the most human. He's the most like us. He can be kind of broken. Which is really fascinating to be coupled with all this heroic stuff. Reporter: The movie pits the two mightiest icons of the dc comics universe against each other. Stay down! If I wanted it you'd be dead already! Reporter: "Batman vs. Superman" is the -- wait, what? Of movie titles. It's counter intuitive. You think, a, they're both good guys. And B, how could Batman fight Superman since Superman is an alien and way stronger and invulnerable? If you'd Ra have a head to head, Batman vs. Superman punchup, we know who wins. Reporter: "Batman vs. Superman" uses multiple story strains Jesse Isenberg's string-pullingvillian to put the do-gooders at odds. Starting with a reprise of the end of "Man of steel" and its destruction of the metropolis. You do have to realize that there is collateral damage which happens and someone's going to get blamed. That someone happened to be Superman. It was important to sort of see that, yeah, there's consequence. It would be amazing if Bruce Wayne witnessed that. He has the power to wipe out the entire human race. If we believe there's even a 1% chance he's an enemy we have to take it as absolute certainty and we have to destroy him. Reporter: He takes on 32-year-old Henry Cavill's Superman Clark Kent. Affleck at 43 is the oldest ever to be past at Bruce Wayne. I wanted a Batman that had been Batman for 20 years. He had the experiences that we know sort of collectively in pop culture. I felt like that character had a chance against Superman. Because he could outthink him. I wanted to be this one of these movies that was ballsy and had something to say. I wanted to do a movie my kids would think it's cool. My son thinks it's cool. Reporter: Samuel, his youngest, led Affleck to meet the actor who excelled in the three Batman epics who elevated the genre and grossed more than $1 billion Tom stick. I was in literally a costume shop. Not only that but my son, really into Batman, wanted a Batman costume. I was in the Batman aisle. I hear, Ben, is that you? I turn around and that's my bad immepersonation of Christian bale's voice, who I didn't even realize was british, such a good actor, I'm like, wait a minute. You don't talk like that. It's Christian. And he's incredibly sweet, really cool. And he's talking to me about going off and doing the movie. 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. Reporter: Inside the suit a physique painstakingly primed to superhero standards. An absolute necessity given kaville's instagramcally impeccable pecs. That was daunting. I thought, if I have to be in as good of shape as this guy I'm in trouble. I worked out for almost a year before this movie started to get into becoming a superhero at my advanced age. And that does not come easy. That's what audiences have come to expect. I look at the Adam west Batman. I think, god, those guys got away with murder. Reporter: Affleck should know. He was in his less-muscular mid-30s when he put on the super suit to play TV's man of steel star George reeves in 2006's "Hollywoodland." I need to go and have a look at that, maybe I can make fun of Ben on the press tour. Reporter: "Batman vs. Superman: Dawn of justice" cues fan boys and girls the director Snyder is looking to amass the justice league, dc's all-star array for future avenger-style films. With this butt-kicking super sylph making her big-screen debut. She with you? I thought she was with you. My most fun day is when my kids came to set and saw us in the costumes. My son was like, is that the real wonder woman? I was like, yes! That actually is the real wonder woman! You're finally seeing the real one. How about when kids see you? What's it like when a child comes across and recognizes kind of who you are? What's that like to be on the other earned of? That is probably the scariest part of this. Because when a child sees you as Superman, there is a lot of responsibility there not to mess up. You're often not expecting it. It's the last minute a parent thrusts their child in front of you and you have to say just the right things. You never know what kids are going to ask, those kids are honest. Reporter: Audiences expected to flock to its March 25th opening will be honest as well. With Snyder looking to show "Batman vs. Superman" is a worthy success tore Nolan's films. And Affleck eager to provide more than just moral support as his director turns 50. Are you going to do anything special for him in honor? I'm going to give him a lap dance, yeah. It's the only gift I can be sure he won't give back. And he can't regift. Reporter: For "Nightline" I'm Chris Connelly in Los Angeles.

This transcript has been automatically generated and may not be 100% accurate.