The two funnymen talk about being siblings in Step Brothers

In Step Brothers, Will Ferrell and John C. Reilly portray the unbelievably irresponsible adult children of Mary Steenburgen and Richard Jenkins, who have to learn to live with each other when Ferrell's mother and Reilly's father get married. And when their ridiculous antics threaten their new family, the two are faced with a fate worse than having to accept each other as step-brother: they are finally going to have to get jobs and move out.

The two previously worked together with Step Brothers director Adam McKay on Talladega Nights: The Ballad of Ricky Bobby, and it seems the men enjoyed the experience so much they conceived of Step Brothers simply as a way to work together again. "We had so much fun working on Talladega Nights, the three of us, and we really kind of made a pact, a blood pact, to work on something else together," Ferrell jokes. "I think it was John who was really the catalyst, who said, 'Let's really make a concerted effort.'"

"You work on films with people, you have fun experiences, and then you say 'Let's do it again' and then it slips through your fingers," he continues. "We really made a point to sit down and meet. I think we had a couple of dinners where we threw out a bunch of ideas and had some really good ones. And it was Adam who called both of us the next day and said, 'I just thought of this other thing: What if you guys are two 40-year-old guys who live with your single parent? They meet each other, get married, and you're forced to be step-brothers.' We were both like, well that's the idea."

It takes less than a minute in a room with Ferrell and Reilly to see how much they like each other, and how genuinely funny they are off the cuff. So with two such gifted improv comedians, the big question is how much of the movie was scripted and how much just came up as the cameras were rolling?

According to Reilly, a lot of what you see is true to the script, which was co-written by Ferrell and McKay. "Sometimes you'll do films and they'll be like, 'You'll improvise something funny here,'" Reilly says. "And you read the script and it's just a weakly written scene. The reason you want improve is because you didn't do the work. But these guys, the script is always really, really funny. It's kind of sad to move away from it because it's like, 'Damn! This is so good!'"

"We improvise a lot in the writing process, so even our written scenes have a back and forth feel that make it seem improvised," Ferrell explains. "When we write, we have a trunk of silly hats and wigs. We'll put them on and have a small troupe of actors that stay in a room. And we'll call them in when we want to act out some scenarios," he deadpans.

With all that improv, you can bet there are going to be some great deleted scenes and extras on the DVD when the time comes, including a sleepwalking scene with Reilly and Ferrell, as well as a sex scene between Reilly and Kathryn Hahn, who plays his sister-in-law. "During the Christmas dinner," Reilly says, "she writes on her plate, 'Come outside with me right now' or 'Meet me in the room,' or 'Let's "f" now.' We go into the other room and we're having sex right outside the doorway, like, while they're continuing the scene. And we keep falling into this two-way door, and I pretend like I'm helping her with her back. It escalated to the point where she was spread-eagled with her legs out and we fell over and I claimed to be doing a yoga pose with her."

Perhaps most notable about Step Brothers is the rather R-rated nature of the comedy, which is different from a lot of trademark Will Ferrell movies. "It wasn't like we were deliberately trying to be R-rated," Reilly explains. "We just didn't want to have constraints. We know how creative it gets when you start improvising and coming up with ideas. We just wanted to feel like it could be anything we wanted it to be."

Step Brothers opens nationwide on Friday, July 25.