When director Clint Eastwood and actor Bradley Cooper first set out to turn Navy SEAL sniper Chris Kyle’s autobiography into a Hollywood film, the pair traveled to Hamilton, Texas to meet with Kyle’s father.

The duo hoped to learn about Kyle’s life, and sought the blessings of his family before taking on the arduous task of creating a film — American Sniper — that would pay tribute to the legendary hero.

Wayne Kyle, Chris’ father, had a few reservations about the project when he met with Cooper and Eastwood, and it was reportedly an intense meeting.

The elder Kyle said he looked directly at Eastwood and warned him: “Disrespect my son and I’ll unleash Hell on you.”

Both Eastwood and Cooper assured him they wouldn’t do anything to cause the family any anguish.

“They looked me in the eye and told me they would honor my son’s memory. That was good enough for me,” Wayne said. “Both Mr. Eastwood and Mr. Cooper were men I could trust. They were very open and engaging and did not want to hide anything.”

The father had his doubts about his son’s 2012 autobiography being turned into a movie because he knew “what Hollywood can do and how it can change things.”

He was convinced by their decision to personally come see him and how “very respectful and honest” they were.

Cooper is still able to recite the names of Kyle’s entire family from memory, and described Kyle’s home as “sacred ground,” he said:

We spent three days with Taya, Kyle’s wife, and McKenna and Colton, the two children, and then Wayne and Debbie, his parents, and Jeff and Amy, his brother and his brother’s wife.

It changed us for the rest of the shoot. It was the thing that sort of concretized it for us and really made us both realize just how important a responsibility we had, and we also got tons of material from it.

American Sniper tells the true story of the most lethal sniper in Navy SEAL history. Kyle served four duties in Iraq, from 2003 to 2008, and according to the Daily Mail, killed as many as 255 insurgents.

Kyle was murdered in 2013 at a shooting range in Texas while assisting a veteran struggling with PTSD.

American Sniper will debut in the United States on Christmas Day.