The new film centers around a group of six friends who reunite to play The Buddy Games, a competition in which everyone participates in an assortment of wild and wacky challenges. In one part of the film, Bobfather (Duhamel) battles it out with Tiffany (Munn), who absolutely kicks his butt!
"That was one of my favorite moments," Duhamel admitted to ET's Lauren Zima. "She's like a Kung Fu master, that one. She's real and that's all her by the way. She actually could kick my a**."
Duhamel wrote, directed and stars in the stunt-filled bromance, which is based on his real-life friendships. In addition to Munn, Duhamel also got Dax Shepard, Kevin Dillon and Jensen Ackles on board, to play Durfy, Doc, and Jack, respectively.
"I learned a lot making this movie," Duhamel explained. "Every step of the way was a learning experience for me. Casting it was tough because we didn't have a lot of money. I had to call out a lot of favors."
"These guys all basically did it for free," he continued. "I am going to owe a lot of them favors after this, but they showed up on a friendship and truly laid it down. They were fearless about it, and I am just really proud of the movie. I am just happy to unleash it, and I think people are going to love it."
Duhamel revealed to ET that the various competitions you see in Buddy Games are based on challenges he's had with his friends in real life.
"When we get together it's everything from ping pong to wiffle ball to golf, and then we do something crazy every year," he explained. "Whether it's go-kart racing, or this last year was target practice. You had to stand there in your underwear with a helmet on and take paintballs, and if you flinched you lost points."
"So, it's crazy," he added. "We immediately revert back to our 15-year-old selves when we get together. I'm not proud of it but it's OK to keep that kid alive inside of you."
Despite all the craziness that ensues, Duhamel hopes that viewers will understand the film's underlying message about the importance of friendship. It's something he hopes to pass down to his own son, Axl, whom he shares with ex-wife Fergie.
"That's really the overall message to this. I think it's overshadowed a little bit by the craziness, the extreme nature of a hard-hitting comedy that this is movie is, but it is about friendship," he shared. "It's about brotherhood and it's about camaraderie and how important those things become as you get older. For me, a lot of these guys I've been friends with since I was in kindergarten, so I really value these friendships. We're hard on each other but I think anybody who has a group of friends that they've known forever would probably say the same thing."
As for 7-year-old Axl, Duhamel says he's already "a very social little dude" who has "no problem making friends."
"I would imagine he'll probably be in the same position I am when he's my age," he revealed. "He's a friendly kid and I think that he already recognizes how important it is to have friends."
"It's important, especially now with everything going on. I think everybody kind of misses the old times and getting together with their buddies, and I think that this movie couldn't have come out at a better time, right before Thanksgiving," he added. "Axl and I are gonna be here [in Los Angeles] for Thanksgiving this year, it's gonna be a little bit different, because normally we're back home. And then Christmas, we're gonna go to the cabin in Minnesota, and snowmobile, sled, build fires, watch movies."
The Buddy Games is available now on demand, digital and DVD.