The pair co-star in 'Flag Day,' which is due out Aug. 20.
Dylan Penn enjoyed working with her dad! ET's Lauren Zima spoke to Sean Penn and his 30-year-old daughter at a Los Angeles screening of their new film, Flag Day, and the younger Penn admitted that working with her dad was an "intense" experience, but one that she'd happily do again.
"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. "To be that vulnerable is scary, so to do it with your family around does help."
For Sean, the movie, which is based off of the real-life story of a father, John Vogel, who lives a double life as a counterfeiter, bank robber and con man in order to provide for his daughter, Jennifer, was 15 years in the making.
Sean stuck with the project, which he also directed, in large part due to its "sensational script." For each of those years, Sean 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 said. "I gave her little nudges over the years, she had other nudges."
Katheryn Winnick, the father-daughter duo's Flag Day co-star, was quickly taken with the pair's "beautiful" relationship, both on screen and off.
"I have known Sean for many years and I have known Dylan before, just seeing them as father and daughter is just so special, but then also seeing them on screen and having that translate, I think the movie would not be as touching if it wasn’t the two of them," she told ET. "I can’t imagine anyone else playing Sean and Dylan's characters."
Sean agreed, and offered his daughter loads of praise for her performance.
"The very first time I read [the script], Dylan's face was imprinted in my mind as Jennifer," he said. "That's been the driving dream. Now she came and made the ultimate dream come true in terms of her talent. What she is able to convey on the screen is so unique and beautiful."
"She was so extraordinary," the proud dad added of his daughter. "It was such a gift to be able to have this experience with her... I think she captured something so [full of] magic here that I can't imagine not having done it at this point."
Dylan felt similarly about the project, telling ET, "I think I'm still processing everything that's happening right now, but I think that I'm pretty aware of the fact that when I look back on this, however many years from now, that it will be a special moment in time, for sure."
Flag Day will hit theaters Aug. 20.