Speaking with ET's Kevin Frazier, Gyllenhaal opened up about the paternal instinct that informed his Southpaw character, explaining, "I am not a father yet, but it is definitely a dream of mine, should I be lucky to have that to happen."
"I have nieces who I love deeply and I have children in my life I love deeply, but when I meet Oona for the first time, I just fell for her," Gyllenhaal shared. "Your heart opens up to her."
The actor said that his young co-star actually helped him develop and guide his character and his performance.
"When I started acting with her too, she was the one who was guiding me. We improved a lot, and she would make choices, I would just follow her in an improv, and that's the dynamic," Gyllenhaal recounted.
Check out the video to hear more from the celebrated star about his super-talented co-star, and why he wants to be a dad.
Southpaw hits theaters July 24.