The Dark Knight star underwent a heavy make-up session so he could be aged as the revered Texas Longhorns legend, and admits he saw a lot of his father in the film.

"I felt like it was a pretty good representation of what I'm gonna look like when I'm old," he tells WENN. "I saw myself in it but I also saw how my face was gonna go and my hair; I saw a lot of my dad in that. Also that I really need to use good skin care products! My face is already falling off; I have so many wrinkles."

The actor reveals the movie was a lot tougher than he thought it would be - because he'd be upsetting so many loyal Longhorns fans if he screwed the film up.

"There's so much pressure doing something that's real," he adds, "especially with all the Twitters in our world because I'm gonna get a lot of blowback: 'He did this, he did that...' There's a responsibility.

"Darrell Royal basically pioneered the triple option and the wishbone. He won three national championships for the Texas Longhorns. His name is on the stadium in Austin and there's an oil painting of him in a restaurant. When I went out to eat my first night in Austin, I'm eating a piece of steak and look up and there's the oil painting. I go, 'Oh c**p. I really gotta study!'

"People would come out of the woodwork when I was on the streets and give me their experience they had with Royal. Plus, there were former players watching us practice and watching us film. You wanted them to nod their head, not shake their head. And me coming from the west coast and not having an accent and not being a part of Texas football - I guess Matthew MCConaughey was busy... I felt the weight of that."

But Eckhart only needed the approval of Royal's wife Edith to know he'd done a good job: "I went to dinner with Edith and she talked about him... They were royalty.

"One time on set, when I was giving a speech to the players, she was there. When we finished, she was in tears and gave me a nice word and that meant a lot to me. You don't want to screw it up. You want to respect that privilege and want to have their blessing. Movies are influential and they can go a long way for good or for bad so I take that seriously."