Actor Bradley Cooper is quick to say that "American Sniper", his film about Navy Seal sharpshooter Chris Kyle, is not about the Iraq conflict but an intimate character study of the harrowing impact of war on a soldier and his family.

The dual Academy Award nominee, who is also a producer of the film, directed by Clint Eastwood, had planned to collaborate with Kyle to bring his story to the big screen.

But Kyle, a former rodeo rider, survivor of four tours in Iraq and the most lethal sniper in Navy history, was murdered near his Texas home by a disgruntled veteran before the two had a chance to meet.

"It is a movie about what someone like Chris, a soldier, has to go through and the dilemma and the horror of it and the battle internally and with the family," Cooper said about the film that opens on limited release on Christmas day and nationwide on Jan 16.

Although Cooper never met Kyle, his wife gave the actor access to personal emails the couple had exchanged during his tours in Iraq and family videos that he found invaluable in finding the essence of the man.

"She opened up her life. We didn't have to create anything with our imagination, literally nothing. All we had to do was soak in what she gave us," Cooper, a best actor nominee for "American Hustle" and "Silver Linings Playbook", told a news conference.

The film shifts between battle scenes in Iraq, flashbacks to his childhood, and difficult homecomings as the couple try to cope with the impact of Kyle's experiences on their family.

"It is important to show their relationship, to show the dilemma that Chris faced. It is his story," said Sienna Miller, who plays Kyle's wife in the comeback role.

Cooper, currently winning rave reviews on Broadway in a revival of "The Elephant Man", about the life of a disfigured 19th century Briton, bulked up for the role and trained with live ammunition to get into the mindset of Kyle.

He said he hopes the film will open viewers' eyes to the struggles soldiers face, in battle and when returning home.

"The takeaway will be, for those who can relate to him, will be healing, to relate to a vet who has gone through similar things that Chris has gone through, and maybe not feel so alone," he said.