updated 01/13/2010 AT 12:00 PM ET
•originally published 01/14/2010 AT 12:10 PM ET
Mark Wahlberg is no stranger to pushing the envelope when it comes to movie roles. In his latest film, The Lovely Bones, the actor – who plays a father trying to cope with his daughter’s murder – went far beyond his comfort zone, forcing him into a state of mind he hopes never to experience in real life.
“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.