Josh Hartnett Became a 'Hardcore Swiftie' with His Daughters: Guess the Album They Like Most (Exclusive)

Josh Hartnett became a Taylor Swift fan thanks to his daughters.

In the twisty new thriller Trap, the actor, 46, plays a dad who brings his daughter (played by Ariel Donoghue) to a major concert that's intentionally similar to one of Swift's shows. Here, the fictional pop star on stage is Lady Raven, brought to life by Saleka and a soundtrack of original songs.

Hartnett tells PEOPLE it wasn't until after he filmed the movie that he finally attended Swift's Eras Tour, bringing his daughters to a stop in London.

"I became well-versed after the movie, but before I really didn't know," Hartnett says of the real-life fandom and "mania" surrounding Swift, 34. "Some of the stuff, while we were filming, I was thinking, 'God, this is a little over the top. The amount of the fervor of the audience, and they know every word.' Then I went to the concert. I was like, no, it's exactly right."

Hartnett shares four kids with wife Tamsin Egerton, the eldest of whom turns 9 later this year. He says gearing up for their big concert meant listening to Swift together on a loop.

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"The thing is, my kids weren't really into pop music so much last year when we shot [Trap]," he says. "Then when they heard the Eras Tour was coming because their friends at school were all into the concept, they just immediately became Swifties — and really hardcore Swifties."

"We listened to that music for six months nonstop in the car before we went to the show," says Hartnett.

What's their favorite of Swift's eras? "The album that they play a lot at our house is 1989, and I like that album. It's a good album."

In addition to being big-time Swfities, Hartnett's kids are currently into horses. "What's fun about being a dad is that your kids introduce you to new things that you didn't know you'd ever be interested in. You end up doing things that you never would do for yourself," he says.

"Right now my kids are very into riding horses, so I'm learning about that stuff," adds the actor. "I'm excited to learn more things [with them]."

Trap is in theaters now.