The 40-year-old actor covers Attitude's August 2018 issue, and inside the magazine, he opens up about reuniting with his ex-girlfriend for the upcoming Mamma Mia! sequel -- admitting that while it was a "delicate" situation, it was "nice to spend time" with her again.
Cooper and Seyfried started dating in 2007 after meeting on the set of the first Mamma Mia! film, in which they played an engaged couple. They split in 2011, but reprise their roles as Sophie and Sky, who are now married, in the sequel.
"She's a friend," he explains. "It was always going to be OK. You just have to be delicate with those situations."
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Go Behind the Scenes of 'Mamma Mia! Here We Go Again' (Exclusive)
"She's married now and has a beautiful child," Cooper continues, noting Seyfried's 2017 wedding to Thomas Sadoski, weeks before she welcomed their first child, a baby girl. "We knew we were going to be working together [again] on a set, going back to a part which was certainly going to remind us of 10 years earlier, but I know her family, I know her mum, who was there a lot."
"It was a pleasant atmosphere," he adds. "It was really interesting to see how different our lives are after 10 years. It is a long time and it was nice to spend time [with her]."
In fact, it seems it was all fun and games on set of the sequel, as Cooper revealed during his Wednesday appearance on The Late Show With Stephen Colbert that their co-star, Cher, teased them a bit about their past relationship.
"She didn’t trust me at all," Cooper said of the legendary singer. "She would tell Amanda once she found out that we used to go out together once upon a time and say, 'You dodged a bullet,' in a nice way."
Seyfried, meanwhile, recently told The Mirror that while she and Cooper had remained friends after their breakup, her husband, Sadoski, wasn't exactly convinced Cooper's feelings were in the past.
"I think he has the same kind of issues that I would have if we were hanging out with his ex-girlfriend from long ago," she shared. "I think it's always like, 'I'm so in love with this person, so that person must be so in love with her too.'"
"I'd guess he assumed Dominic is in love with me the way he's in love with me. And it's just not the case, it's sweet," she continued. "I'd rather he be jealous than completely fine with it."
ET was with Seyfried, Cooper and the rest of the Mamma Mia! Here We Go Again cast at the film's London premiere on Monday. Watch below.