Tonight you're going to meet an unlikely action hero. Chris Pratt has a cult following, I'm a long-time fan. He is certainly not a household name. So how did he land a role in what could be one of the... See More
Tonight you're going to meet an unlikely action hero. Chris Pratt has a cult following, I'm a long-time fan. He is certainly not a household name. So how did he land a role in what could be one of the summer's biggest movies? Here's ABC's Chris Connolly. My name is peter quill, okay? Dude, chill out. Reporter: It's got a shambling, self-deprecating, hero dude peter quill. Starlord. Who? Fstarlord, man. Reporter: Leading a ragtag multi-species team on a bring me the orb mission. When you meet these characters the last thing you're expecting them to do is save the galaxy. They're thieves, they're unpredictable, they're very dishonest. We're not your stereotypical super heroes. I think people will be able to relate to us. We've already established your destroying the system is not saving me. What did I destroy? Like three seconds ago. I was thinking of something else. Reporter: They help make "Guardians of the galaxy" set away from the planet Earth. It's got the full backing of Disney which owns ABC and the a-list participation of Bradley cooper. It's real! Totally fake. That is the most real, authentic, hysterical laugh of my entire life. Reporter: And vin diesel as a tree. If you would have asked me 20 years ago that I'd come to Hollywood and end up being nature's ambassador? I would have laughed. You've heard of this, you've seen this, you know what that is. Reporter: With all that, this entire enterprise rests on the broad shoulders of an actor who is hardly a household name. This guy. Chris Pratt. Did you ever think you would be this guy? Yes. My whole life. I was just waiting for everyone to catch on. Like, you guys, I'm a superhero, put it together. No. Frankly, when somebody first suggested him to me, I thought, you've got to be kidding me, the chubby guy from "Parks and rec"? No way I'm going to make that guy the star of my movie. Where's all the faces of the presidents? Reporter: On "Parks and rec" he's beloved as Andy Dwyer. Did you know the food you eat becomes energy? Yeah! That's spaghetti. Nachos. Cookie. That's my husband. Reporter: Now 35-year-old Pratt and his appealingly underachieving persona are being asked to set the tone for marvel's first film not based on a wildly popular comic. Can he do it? Well, he needed less than a minute to convince one skeptic. I knew within 20 seconds of his audition, this is absolutely the guy. They show you a script, did you get to read -- No, marvel did not show me a script. They're like, hi, we're marvel. I was like, I'll do anything. They're like, yeah, you will. Chris Pratt is the biggest movie star in the world, people don't know it yet. He as big, masculine dude in the way few movie stars are these days. Reporter: Not many can French braid hair as he does at home for his spouse. But before shooting "Guardians" all agreed he needed to tone up a bit. Send me prop guns. Take a picture with it, pose with the guns. Secretly they're doing, how fat is Chris? That's what the photo was. Oh, we need you to try on these gloves, full-body shot with the glove on. He'd tweeted that photo of himself from "Zero dark thirty" so we knew he could do it, get into that kind of shape, we trusted he could. He did and got into even better shape. They sent someone to my house to measure me. Like I was on "The biggest loser." Reporter: With ongoing "Star trek" and avatar" roles Zoe Saldana has more franchises than a food court. For this film she used her ballet skills. It's everything. It's the reason I'm able to do this. How did you help shape what you look like? I didn't use the right pg-13 word, but I want fan boys to go, she's hot, she's beautiful. Green is a hard color to sell as kind to the eye. I was like, can we pad these up a little more? I was very technical about the whole thing. Oh, yum! Reporter: There was no padding vin diesel's dialogue. Just three words. I'm Groot! Reporter: Spoken again and again. The action star got the joke. When they called to ask you to do this, you would have been well within your rights to like throw the phone over the fence. Yeah. Wow. First person I told had that kind of reaction. Where it was like, huh? No, no, no. Then everybody's waiting for you to do a marvel. And I immediately fell in love with the character. Three words at a time. Three words at a time. I'm Groot! He was very sious about the character of Groot. He must have said at least 500 "I am groots." Jump cut to my kids seeing the movie. Now whenever we're driving and they see trees they refer to the trees as daddy's brothers and sisters. Yeah. Life is good. ? Reporter: As "Guardians of the galaxy" rides positive reviews into theaters August 1st, no one has more reasons to feel good than U.S. Unlikely star. Who? Starlord, man. Legendary outlaw. Guys? Oh, forget this. I used to be a door-to-door salesman, I lived in a van, man. I'm a Hollywood movie star now. I'm going to be happy every day I walk on the set. I have nothing to complain about. How fitting for you that in the pilot of "Everwood," you rip up a comic book. Whoa! Chris! My mind is blown. That's right, I went from that guy to this guy. Whoa, that's true, I do. I rip up a comic book. In this, I rip up a comic book movie. Reporter: I'm Chris Connolly for "Nightline" in Los Angeles.
This transcript has been automatically generated and may not be 100% accurate.