"It's great when you have a book that a movie is based on because you have so much more insight into the characters," he said.
Bridges added that the straight-talking, heavy-boozing U.S. marshal is far from his actual personality, but the actor tried to focus on how Rooster would be feeling -- and walking -- while under the influence.
"I don't drink like Rooster," Bridges said. "I've certainly been, you know, drunk before in my life. And I don't smoke the cigarettes like Rooster, but I have smoked cigarettes. So I have personal experience with those things. ... You kind of mess around with that."
A father to three daughters and soon to be a grandfather, Bridges said playing the protector of the young, fearless Mattie Ross, played by Hailee Steinfeld, came naturally to him.
"She reminds me of my daughters in that she's so sweet," he said. "It felt like she was another girl, my other daughter."
Bridges has been an outspoken fan of 14-year-old Steinfeld, who also has an Oscar nomination for the film.
He said he didn't give his young co-star too much acting advice on set, except to repeat what his wife, Sue -- and his own mother before her -- always says to him as he leaves for work: "'Have fun and don't take it too seriously,' I probably said that to Hailee."
He admitted he has to take his own advice sometimes.
"It's a challenge for most of us," he said.