Matt Damon on Why Working With Ben Affleck on 'The Last Duel' Felt Different From the Past (Exclusive)

Matt Damon reflects on how his and Ben Affleck's lives have changed since they burst onto the scene with 'Good Will Hunting.' 

Matt Damon is opening up the experience of re-teaming with his best friend, Ben Affleck, on The Last Duel. ET's Kevin Frazier spoke to the actor about his new film, Stillwater, and he also spoke about how his and Affleck's lives have changed since they burst onto the Hollywood scene with their 1997 Oscar-winning film, Good Will Hunting

The Last Duel's first trailer dropped on Tuesday and featured his and Affleck's highly anticipated onscreen reunion. Damon plays the revered knight Jean de Carrouges, while Affleck plays Count Pierre d'Alençon. When de Carrouges' wife, Marguerite de Carrouges (Jodie Comer), accuses his close friend and squire, Jacques Le Gris (Adam Driver), of rape, the matter is settled with a duel to the death. Damon tells ET what it was like working with Affleck this time around.

"I think that writing process for Good Will Hunting was so inefficient," he shared. "You know, because we didn't really understand structure so we wrote thousands of pages. ... We'd be like, 'Well, what if this happened?,' and then we'd just write different scenes. So, we had all these kind of disparate scenes and then we kind of tried to jam them together into something that looked like a movie."

"But this time around, you know, it's a story about perspective," he continued. "So, there are two knights and then there's the Lady Marguerite. So Ben and I wrote the male perspectives and Nicole Holofcener wrote the female perspective. That's kind of the architecture of that movie. And I think we just found that having made ... like, making movies for 30 years, we actually learned something about structure along the way and the process went along a lot faster. And so I think we'll write a lot more in the future just because it didn't turn out to be as time consuming as we thought. It was actually a lot of fun."

Damon says that since he and Affleck are now both fathers, how they spend their time has changed accordingly.

"Back in the day, we didn't have deadlines because nobody cared what we were doing, no one was waiting for the script, we were unemployed, so we literally had nothing else to do," he recalled. "And now we can build the time, it's a little more structured, right? Like, alright, let's write from 10 to 2, you know, because we can drop the kids off and then we can pick the kids up. We actually have lives now which is nice, finally."

Meanwhile, in Damon's latest film, Stillwater, he plays a father who travels from Oklahoma to France to help his estranged daughter (Abigail Breslin), who is in prison for a murder she claims she didn't commit. The movie is loosely based on the real-life story of Amanda Knox, though Damon says it's only used as a jumping off point.

"I think what Tom [McCarthy] our director was really interested in was like, what happens after all the cameras go away and what happens to that family," he shared. "And then he just started thinking about it as what if the father was a roughneck from Oklahoma, you know, because that's a very specific thing... All the workers there, like, they don't usually leave those geographies kind of where they grew up. And what happens when you transplant someone like that into a city like Marseille, which is its own thing and so specific and different? And this guy's basically got none of the skills that he needs -- he doesn't speak the language, doesn't really know the legal system in France, and yet he's trying his best to help his kid."

Damon spent time with oil rig workers from Oklahoma in preparation for the role, and shares their skepticism of him at first.

"I think they were justifiably wary of, you know, Hollywood people coming in and making a movie about a roughneck," he said. "They're like, 'What are your intentions here?,' because you know, these guys can get stereotyped in movies or are the kind of people where, like, the cultural elite kind of look down their nose at. And I think once they realized what Tom and I were doing and that this is a story and a script that has incredible compassion and empathy for this character, they were amazing. They just gave us access, they took us around to their oil rigs and where they work, showed us what that was like, I met their families. I spent a lot of time just hanging out, riding from one place to another in Oklahoma because nothing's too close in Oklahoma -- big, big state. And it was just a really wonderful experience."

Stillwater hits theaters on July 30.