Inside Sam Shepard's Final Years and Why He Kept His ALS Battle Private

Even as he battled a debilitating disease in his final years, Sam Shepard stayed hard at work — as only close colleagues knew of his health struggles.

The acclaimed actor, playwright and writer died last Thursday at 73 of complications from ALS, a representative for his family said. He fought the neurodegenerative disease for a “couple years,” his longtime editor at Knopf, LuAnn Walther, tells PEOPLE.

“Sam was very private and shied away from publicity, and hence he was quiet about this too,” Walther says. “Even in these last years when it was hard, he was just constantly writing.” He released his final work of fiction, The One Inside, in February.

A source tells PEOPLE the star wanted to keep his battle with the disease under wraps in his last year as he lived “peacefully in Kentucky,” where he died.

The source last saw Shepard at a party in late 2015 alongside his longtime friend Patti Smith. Although the disease had already started to take its toll, the source says Shepard was as magnetic as ever.

“He was walking with a cane, but he looked good,” the source recalls. “He was talking with people, really friendly. He looked like a movie star. He was so charming.”

Even though he never spoke about it, Shepard may have given the public an insight into his private struggle in his last book. In a passage in The One Inside, he wrote about a man who couldn’t get up from bed in the morning and felt as though his limbs weren’t connected to “the motor” driving his body.

“It’s so cold. Something in his body refuses to get up,” he wrote. “They won’t take direction — won’t be dictated to — the arms, legs, feet, hands. Nothing moves. Nothing even wants to. The brain isn’t sending signals.”

Shepard with his longtime partner Jessica Lange and their two kids Hannah and Samuel (right).

Director Camille Thoman says that Shepard didn’t seem sick while they were filming his yet-unreleased final movie, Never Here, in 2015 and 2016 — and he inspired the cast and crew whenever he stepped on set.

For much more about Shepard’s legendary life, career and romances, pick up the next issue of PEOPLE, on newsstands Friday.

“Shortly after the film wrapped it became clear [he was sick], but while he was on set I had no idea,” the director says. “He had a tremendous impact on our set and on me personally because he’s such an extraordinary person to be around. He definitely set a bar. Having him around made everyone want to do their very best.”

• Reporting by MIKE MILLER and KARA WARNER