Transcript for 'Avengers: Infinity War' stars on the new film, working together through the years
?????? The story's like a freight train. That's what kind of blew me away. I saw it and I'm covered in goose bumps right now, I'm not even joking. Reporter: Brimming with battles and story-telling ambition, with marvel superheroes of every make and model slugging it out. As much as I complain about it, about getting the crap kicked out of me, I also love it. I love the grit and the grunt of it. I like a dirty fight. I like it, I like to watch it, I like to participate in it. We don't want to kill you, but we will. Reporter: Marvel's "Avengers: Infinity war" spans a multi verse with 19 names on the poster, more than 30 marvel men and women in the mix, the 19th film in that cinematic universe. We're going to show our conduct to the rest of the world, not what I imagined. Reporter: From black panther to a New York City school bus. From the expanse of outer space to inside Robert Downey jr.'s iron man outfit. You're rocking a new suit as well? You're damn straight, come on. What good am I if I don't? Reporter: "Infinity war" marks the culmination of a movie-making plan hatched 10 years ago on the shoulders of a lesser-known Tony stark. Go team, yeah! Reporter: Downey helped inaugurate the marvel cinematic universe of interlaced films and characters. 2008's "Iron man." Oh, yeah, whoo! Reporter: Getting cast as a superhero was a game changer as the rest of this avengers core four shared with us. How much did his involvement affect your willingness, desire, to be in these movies? 100%. 100%. Yeah. He created a space in a world where actors like us generally weren't invited. When he was cast as iron man, when Robert Downey Jr., the indie genius, was cast as the lead, there was now a space for us in the studio system. And he opened that up for us. It's been a -- I think a gift that I never could have anticipated when I first signed on to do "Iron man 2" with Robert 10 years ago. It's I think an experience that -- I still remember the camera test. I remember everything. I remember everything too, Robert. Reporter: The opening weekend of "Infinity war" figured to be remembered as one of the biggest in movie history. Further vindication of marvel's long-standing approach to put its passionate fans first. Like okay, what's true to the comics, what do the fans love, didn't love, how can we evolve, how can we make this better, how can we make it unexpected? To have that following of fans stick with us through all of this is unlike any experience any of us will ever have again. Reporter: Without serving up spoilers on plots or fates of the mighty, safe to say the avengers mix with the guardians of the galaxy. Chris Pratt's star lord tending to Hemsworth's battered Thor. This is a man. A handsome, muscular man. Muscular. Reporter: And it's Josh brolin's epically evil Thanos looking to lay waste to much of human existence. Does put a smile on my face. The key word is Thanos, for all of us, just as actors, we look at it as a catch drain for the nightmare that you hope would never happen. What you end up feeling is just a scope of this thing is so far-reaching. You going to bow? Yeah, he's a king. It's satisfying to bring these different characters together, then sort of have them resolve these strange little conflicts all throughout. What are you doing? I -- we -- we don't do that here. Reporter: For Mark Ruffalo, that can mean issues within his identities. Brainy Bruce banner and -- that guy, the timeless symbol of volcanic rage. One thing in the universe that hulk is afraid of is banner. And so that's a drama that is playing out. Can't you guys just get along? That's a great line. Someone should put that in the movie. Reporter: "Black panther's" success struck a blow for inclusion across the butt-kicking board. Johanson credits marvel's visionary Kevin feige. He's been at the forefront of wanting to put women in the driver's seat where they belong and it's happening, it's very exciting. Reporter: Yet amid all the enthusiasm for the new additions to the marvel mix, it remains enguard for the O.G. And his role. I've heard you referred to as head of the acting department. Yes. I never heard that before. That's interesting. Aptly put. Tell me what you like. Oh, I think I'm good today. No, no, no. Show me. I'm retiring that self-diagnosed moniker. Reporter: Their good humor a further sign that amid change in their lives, these actors have found their reteaming to be a source of strength. Almost like you go to a party, you don't know somebody, then you find a familiar face in a crowd and you stick to that person. I've become a mother since I've been working for marvel. And it's been really grounding to be able to go back to work when maybe all kinds of crazy stuff was going on and I kind of felt like this job was sort of that familiar face in the crowd. This time around I definitely had a sort of nostalgic feeling. Many times we look around and go, oh, wow. Who knows when are we going to do this again, whether we do this again. It's a great sense of appreciation and love for all these guys and this whole journey. I won't even tell you about anyone's art. If I could break the tape and tell you about any one of our arcs, you literally would barely believe it. Reporter: For "Nightline," I'm Chris Connelly in los Angeles.
This transcript has been automatically generated and may not be 100% accurate.