You won't believe what they argued over -- and who won.
The 30-year-old actress revealed that she and her famous dad got into a "two-hour" disagreement while filming the drama. The father-daughter duo stopped by The Late Show With Stephen Colbert on Thursday, where she shared just what they argued over -- and the answer might surprise you.
"We had one major, I think it was a two-hour standoff about a note that he had," Dylan told Stephen Colbert, before sharing what happened. "[It was] about whether I could wear mascara or not wear mascara."
"He won," she noted, with Colbert laughing and quipping, "That is a fight a father usually has with a daughter when she's 12."
The host then asked if they went "someplace private to have this fight, or was it a knock-down, drag-out in front of the crew?"
"No, it was very public," Dylan said, with Sean adding, "Once it was clear that she was making the wrong decision, I just went and sat down, and then the silence was one the crew had to live with for two hours while the Penns figured it out."
Sean both directed and stars in Flag Day, with both Dylan and his son, Hopper Jack Penn, acting in the film as his character's children. Sean plays John Vogel, a father who lives a double life as a counterfeiter, bank robber and conman in order to provide for his daughter, who's trying to uncover the truth about her dad's life.
Dylan previously admitted to ET that while she enjoyed working with her dad, it was "intense."
"It does really feel like you're doing family therapy in front of a crew of strangers that eventually become your family," Dylan told ET at the Los Angeles premiere. "To be that vulnerable is scary, so to do it with your family around does help."
As for Sean, who had been working on bringing the project to life for 15 years, hoped his daughter would join the project, though he "absolutely" did not think it would happen.
"Dylan was so resistant, to the point of rebellious, about the notion of being involved in front of the camera. I was glad and excited when she accepted to do this," he told ET. "I gave her little nudges over the years, she had other nudges."
See more in the video below. Flag Day arrives in theaters on Aug. 20.