"That's what I need to do to get there ... Think about something horrible," Wahlberg tells PEOPLE. "It's not a fun thing to do – that's the thing that bothered me the most."
As a father to children Ella, 6, Michael, 3 ½, Brendan, 16 months, and newborn Grace, Wahlberg admits that experiencing parenthood first-hand gave him the ability to envision the worst and feel emotions he never thought possible.
"I have 13 nieces and nephews and I think, 'Wow, I love these kids like they're my own,' " he explains. "But now, if anyone did anything to harm my kids, as much as I love them, that would be it."
Elated with fatherhood – calling it his "most important job ever, being a dad" – Wahlberg laughs as he recalls the moment he found out his first child would be a girl. "When I first realized I was having a daughter, I was a little freaked out," he says. "But, of course, it's the best thing to ever happen to me."
With the family taking time off to celebrate the arrival of a second daughter, Grace, the break couldn't come at a better time, the actor explains.
"I've been working so much in New York since September," Wahlberg says. "I'm just going to be home and not have anything to do but play with the kids ... and spend some quality time with my wife."
The Lovely Bones is in theaters Friday.
Reporting by ELAINE ARADILLAS