The 34-year-old "Nightcrawler" star, who plays a boxer in the film, said even though his character fiercely fights opponents in the ring it was the scene where he is slapped by his daughter which was "moving" and completely spontaneous, reported Entertainment Weekly.
"She's going through so much and he's going through so much and her ability to be honest with her feelings is what you would want," the actor said.
Gyllenhaal plays Billy, who loses the custody of his daughter, Leila, following the death of his wife, Maureen (Rachel McAdams) and gets a slap in the face when his daughter unleashes all of her pent-up rage toward him.
"Southpaw" also stars Forest Whitaker, and Rachel McAdams.