Sean Penn on the Hilarious Way Matt Damon Persuaded Him to Act Alongside Daughter in New Film

Getty (2) Sean and Dylan Penn (L); Matt Damon

Sean Penn is sharing details about how he got an acting (and family!) lesson from Matt Damon.

The 60-year-old actor was at the Cannes Film Festival this past weekend alongside his daughter and Flag Day costar, Dylan Penn. At the film's press conference on Sunday, Sean revealed he was initially hesitant to star the movie, which he also directed, according to IndieWire.

Sean said he made an exception to his rule — to not take on two jobs on the same project — after Damon, 50, talked some sense into him, IndieWire reported.

"A month and half before shooting started, Matt Damon called me, not to say he could or couldn't, but that I was a stupid schmuck not to take this opportunity to act with my daughter in this thing," said Sean.

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Allen Fraser/MGM Dylan Penn in Flag Day

Samir Hussein/WireImage Sean and Dylan Penn at the Cannes Film Festival

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According to the two-time Academy Award winner, "That was the last straw. Once I decided to do that, it was a big burden off me."

Dylan, 30, stars in Flag Day as Jennifer Vogel, the daughter of Sean's John Vogel — a man who raised her in a fantastical world but has a con-artist past that Jennifer is forced to come to grips with as an adult. Based on a true story, the film is adapted from Jennifer's 2004 memoir Flim-Flam Man: A True Family History.

"When I dreamed from the beginning to make the movie with Sean Penn as John Vogel, to have a real father and daughter play a father and daughter, knowing Sean's commitment to authenticity, this was a special way for this story to be told," producer William Horberg said at the press conference, according to IndieWire. "We didn't compromise. We had to fight hard to get the resources, to get Sean to do this."

"In a film about deception, about the search for truthfulness, it goes back to Dylan's face," said Sean. "I find her uncontrived. When you can put a camera on an actor listening, that tells the whole world of truth and deception. My cinematic focus was on Dylan's face to tell the story."

The film, which made its debut at Cannes, also stars Josh Brolin and Sean's son Hopper Penn.

Allen Fraser/MGM Sean Penn in Flag Day

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Damon was also present at Cannes this year, attending the premiere of his latest movie Stillwater at the prestigious film festival in the south of France. After the movie played, he teared up as the drama received a 5-minute standing ovation from the crowd.

"It was one of those scripts that I looked at it, I finished it, I put it down and I thought about it for 15 minutes. And then I picked it up and read it again," Damon said during a panel. "I just really respond to great writing. It's hard to find these days."

The actor also connected to the central story of a father going all out to save his daughter. (Damon shares four daughters with wife Luciana.)

"Any parent can relate to crossing any number of lines in order to help their child," Damon continued. "Since I've had kids, I feel like everything's a lot more available to me in my job, emotionally speaking. I don't have to reach so far anymore."

Flag Day premieres in theaters on Aug. 20.