March 31, 2016 09:20 PM

Just weeks before Patty Duke died, she was hoping to attend the premiere of a documentary called, Once in a Lew Moon, a film that would ultimately be her final onscreen appearance.

As her health declined in the days leading up to the movie’s premiere, Duke was forced to cancel her appearance. Despite her illness, she still found the strength to write a few notes expressing her gratitude for being involved in the project.

“I had asked her and her husband, Mike [Pearce], to open the film premiere with a speech, and that’s when things apparently took a turn for a worse,” Once in a Lew Moon director Lonnie Senstock tells PEOPLE. “After that exchange, I realized she wasn’t even strong enough to write out what she wanted to say in honor of the film.”

Senstock, who had worked closely with Duke on the documentary since 2006, will always remember the Oscar-winning actress for her grace and perseverance in the face of illness.

“Even though physically she wasn’t able to do what she wanted to do in terms of writing that letter, she still did it and the letter was read,” he says. “And that was really something because we’re talking about just three weeks before she passed.”

He added that Duke “being so helpful and accessible in the middle of being so sick really changed the atmosphere of the film.”

While Senstock knew the actress had been having health issues, her absence from the premiere was especially concerning. In making the documentary, the director says Duke always stood out for her dedication and accessibility.

WATCH: Remembering Oscar Winning Actress Patty Duke

“Patty was the only one that completely, even on set, took the time to be engaged with me, to be in touch, to let me know her schedule, when she could or couldn’t do something,” he says. “The one thing I want people to know is she made a new light as far as saying to people ‘I don’t care how big of a star you are, you can do the projects and work with people on any level, if you’re kind.’ ”

In Once in a Lew Moon, Senstock aimed to shed light on the creative processes of Hollywood artists, focusing on the work of famed screenwriter Lew Hunter. Duke and Hunter worked together in 1971 on a film called If Tomorrow Comes, in which the actress played a student who falls in love with a Japanese-American man at the time of the internment camps.

In the above clip, footage (shot in 2014 by Roger Speakman) shows Duke and her husband walking into an Omaha screening of The Miracle Worker, organized by Bruce Crawford. She is then seen introducing the film to an audience before Senstock rolls footage from If Tomorrow Comes, which is accompanied by an interview with Duke.

“We just had a regular teenage thing going on and then all this horror happens around it when his family is taken away, including him,” Duke is heard telling Senstock. “It was a powerful, powerful experience.”

Senstock, who has made his full interview with Duke available here, says that of all the artists he’s worked with, “Patty was the one that was accessible and the most helpful when other things couldn’t get done.” It’s a trait the director remembers as “incredible as far as being a humanitarian and encouraging other artists.”

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