Sunday October 08 04:46 PM EDT, Yahoo! News

Richard Farnsworth, the unassuming former stuntman whose acting career was jumpstarted this year with an Oscar nomination for The Straight Story, has died of an apparent self-inflicted gunshot wound. He was 80.

Sheriff's deputies confirmed that Farnsworth died Friday afternoon at his rural Lincoln, New Mexico, home. Farnsworth's fiancée, Jewely Van Valin, said he had been diagnosed with terminal cancer several years ago, and authorities speculate that he committed suicide because of the pain.

Farnsworth's career in show-biz spanned some 60 years, but it wasn't until last year that the gentle, grey-mustached actor found himself in the role that would define his career. In David Lynch's The Straight Story, Farnsworth played Alvin Straight, an elderly man who made headlines in 1994 by driving his lawnmower from Iowa to Wisconsin to see his ailing, estranged brother.

The performance landed Farnsworth an Oscar nomination for best actor, and he became the oldest actor ever up for the award. "The minute I read this script, I identified with this old character, and I loved the story," he once said.

But Farnsworth's fiancée said he was in great pain when he starred in the film, and in recent years, the bone cancer had left him partially paralyzed. Van Valin told the Albuquerque Journal she was in an adjacent room Friday afternoon when she heard a gun shot. Sheriff's deputies said Farnsworth left no note.

"He was in incredible pain today. He was going downhill," she said.

Van Valin added that Farnsworth always tried not to show the toll cancer had taken on him. "He was very ill in that movie, but phenomenally, he made it through. He didn't want the world to know he was sick."

His daughter, Missy Farnsworth, told the Associated Press that she last saw her father in June--he couldn't move his legs, but the mood was upbeat, as she and Van Valin playfully drew pictures on his toes.

"He was a very private and proud man who chose not to discuss his health issues," she said. "He had reached a level of pain that he was unable to get beyond."

Farnsworth, born September 1, 1920 in Los Angeles, was a poor student who quit school during the Great Depression to go to work as a stable boy at a polo barn. He was working at the stables in 1937 when scouts from Paramount came looking for ponies to use in the film Marco Polo. After mentioning that they needed extras to play 500 Mongolian horsemen, and that the pay trumped his stable work, Farnsworth signed on.

He later rode horses and doubled for actors in films throughout the 1930s, 1940s and 1950s, including Texas Across the River, Gunga Din and Duel at Diablo. Farnsworth then moved from stunt work to acting at age 57, and landed his first major film role in 1978's Comes a Horseman with Jane Fonda and Jason Robards. The part landed him his first Oscar nomination, for best supporting actor.

Soon, Farnsworth was spinning his down-home charm and kindly demeanor into roles for such films as The Grey Fox, The Natural, Tom Horn, Resurrection and 1991's Misery, in which he played a Columbo-like small-town sheriff.

He was beaten out for this year's best actor Oscar by Kevin Spacey, who won for his role in the Best Picture winner, American Beauty. But Farnsworth received a standing ovation at this year's Independent Spirit Awards, where he took home best male lead honors for the role.

In accepting the award, the then-79-year-old actor took his newfound success in stride. "My agent said that a lot of offers would come through once they see this picture," he said, simultaneously holding back tears and cracking jokes. "I did get one: They need a dialogue coach for Bartles and Jaymes."