The former Heroes star plays Ethan, a young father whose son dies after a tragic turn of events, in the movie, which also features Jeremy Piven and Mira Sorvino.
Dekker, 23, admits he didn't have to look far to convey the deep emotion he was already experiencing as his own parent was losing his life.
But the actor forced himself to separate his personal and professional lives so his own grieving process wasn't played out completely onscreen.
He tells WENN, "That shock (of losing my son in the film) happened naturally... I don't think it's healthy for actors to use personal issues as inspiration. I was hurting so bad on this movie. It was a very painful time because my father was dying while we shot this movie and I realised halfway through that I was using all the personal grief and pain that was not healthy. I had to figure out for the rest of the shoot how to put that aside and focus on the story."
However, Dekker reveals he bonded well with his onscreen kid, played by Ameko Eks Mas Carroll, and emerged from the shoot longing to start a family.
He says, "I really connected with the boy who plays my son. We spent two weeks together all day and every day going to the zoo. He's just this amazing little boy so it was easy to slide into that father role.
"I left this movie wanting to have a child which I never felt before because I fell in love with the little boy playing my son."