YOU can’t be a great actor unless you’re willing to suffer, says acting coach Larry Moss, who guided the previously little-known Hilary Swank and Michael Clarke Duncan through wrenching — and Oscar-nominated — performances.

Swank was ridiculed while walking Los Angeles streets dressed as her character in “Boys Don’t Cry,” while the tears Duncan shed in “The Green Mile” came from his own painful past.

“What actors do is remind us what it is to be human — however horrifying that may be,” says Moss, who lives in Los Angeles. “That’s why we go to the movies.”

A coach for 28 years, Moss has worked with Noah Wyle of “ER,” Helen Hunt (for “As Good As It Gets”) and Jason Alexander of “Seinfeld.” A former actor, Moss turned to coaching at the suggestion of an acting teacher.

“I was 29 and insecure and never thought of myself as a leader,” recalls Moss, “but when I critiqued a scene in my very first class, I felt whole. I suddenly understood what my purpose was.”

When Swank, who’s nominated for Best Actress, went to Moss for help playing the doomed cross-dresser in “Boys Don’t Cry,” her transformation was well under way.

“She’d already taped her breasts, shaved off her long hair and had just pumped iron. She said, ‘Just help me understand, scene by scene, what Teena Brandon wants,'” she said.

Together, they did just that — trying to imagine young Teena’s discomfort at wearing girls’ clothing, and her joy in finally finding herself, as a man.

“It was Hilary’s idea to live as a boy for a month,” Moss says. “She’d go through days where she didn’t pass [as a man] and where she was hated by people who didn’t know what to make of her.

“She would have gone through anything to bring that character to life, because she was so moved by the story. Without her performance, there’d be no ‘Boys Don’t Cry.'”

Castle Rock hired Moss to work with Duncan, a former ditch digger the studio deemed physically perfect for the part of the giant John Coffey in “The Green Mile.”

Duncan was decidedly less enthused.

Just before his screen test, he showed up — all 6 feet, 5 inches and 300-plus pounds of him — at Moss’ Santa Monica apartment and announced, “I don’t really want to be here.”

Nevertheless, the acting coach soon had Duncan talking about growing up fatherless and poor in Chicago.

Duncan told Moss how, at 14, he’d been stopped by a policeman who demanded to know where he’d gotten his new white tennis shoes. Duncan the young teen replied that his mother had bought them for him.

“He told me how the policeman made him walk in the mud,” Moss says softly. “And then we both wept.”

It was that kind of recollection, plus reconstructing a “back story” for Coffey involving a dead grandmother and a lonely, frightened adolescent staying one step ahead of a lynch mob that helped Duncan garner a Best Supporting Actor nod.

“He was so rich, so beautiful, in my apartment that I thought, If he can only do this on film, he’ll get an Oscar,” Moss says.

But Duncan had his doubts.

“How will I ever be able to do this in front of Mr. Hanks?” he asked Moss.

“When he went to the jail cell and his cheeks began to shake,” Moss says, “I knew he was ready. [Director] Frank Darabont said, ‘Action!’ and Michael did it in one take. Tom Hanks said, ‘Oh my God.'”

Come Oscar Sunday, Moss says he’ll do what he always does: sit in front of the TV with a few close friends, eating ice cream and cheering on his favorites — his students.

“The people I coach I hope I help, but I know they help me,” he says. “When they’re as open and raw as Hilary and Michael, I’m a better person because of their bravery.”