Rose Byrne was very careful to hit the right “tone” in her new comedy-drama Instant Family.

The Australian actress stars alongside Mark Wahlberg in the Sean Anders-directed flick, with the plot following a couple who stumble into the world of foster-care adoption and end up taking in three children overnight, including a rebellious teenager, as played by Isabela Moner.

As the film mixes humour with an exploration of more serious subject matter, Rose has now shared how hitting the right balance was important to her.

“Finding the tone is tough,” she told Total Film magazine. “We’re doing different versions of a lot of scenes – pushing the comedy and then pushing the drama. Some scenes are so split down the middle.”

Rose went on to explain that she was attracted to the role because she would be able to tap into her own experiences as a mother, as she and her actor partner Bobby Cannavale are parents to two young sons. But once she began investigating the U.S. adoption system, the star quickly discovered that it was much more complicated than she had thought, especially in regard to the fostering or adoption or older children and teenagers.

“I really didn’t want to go back to work at the time. I’d just had my second kid and I was still recovering from that. But after I spoke to Sean, I put the phone down and I was like, ‘F**k. I’m gonna have to do this,’” the 39-year-old said. “I have friends who were adopted, and I know a bit about the situation in Australia, but I was pretty naïve about the American system. When I started reading up on it the facts just completely floored me. It’s hard to do a film like this and not be permanently changed by it.”

Instant Family, also featuring Octavia Spencer and Tig Notaro, is now showing in U.S. cinemas and opens in the U.K. on 14 February (19).