Inside Robin Williams' Sad Final Days: 'He Was Sobbing' and Couldn't Remember Lines

Robin William‘s final days are coming into clearer perspective.

The legendary comedian — who passed away at the age of 63 in 2014 — struggled to remember his lines on the set of his last movie Night of the Museum: Secret of the Tomb, according to a new biography Robin by Dave Itzkoff.

“He was sobbing in my arms at the end of every day. It was horrible. Horrible,” makeup artist Cheri Minns recalled in the book, according to the New York Post. “I said to his people, ‘I’m a makeup artist. I don’t have the capacity to deal with what’s happening to him.'”

When she suggested he return to his stand-up comedy roots as a means to cope, she said he refused.

“He just cried and said, ‘I can’t, Cheri. I don’t know how anymore. I don’t know how to be funny.’ ”

Williams died after suffering from Lewy Body Dementia, a type of brain disease that affected his thinking, memory and movement control. It’s the second-most common type of progressive dementia after Alzheimer’s disease.

As the book recounts, before his death Williams had been incorrectly diagnosed with Parkinson’s Disease, a progressive disorder of the nervous system that develops gradually, often with a small tremor or general stiffness and slowing of movement.

“I put myself in his place. Think of it this way: The speed at which the comedy came is the speed at which the terrors came,” Minns recalled in the book. “And all that they described that can happen with this psychosis, if that’s the right word — the hallucinations, the images, the terror — coming at the speed his comedy came at, maybe even faster, I can’t imagine living like that.”

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RELATED VIDEO: Robin Williams’s Widow Susan: ‘I Forgave Him’ For Suicide

In an interview with PEOPLE shortly after her husband’s death, Susan Williams opened up about her his struggle with the debilitating brain disease, which had begun taking its toll on the actor in the last year before his death.

Susan explained that although in the months leading up to his death, her husband’s symptoms — which included heightened levels of anxiety, delusions and impaired movement — worsened, doctors weren’t able to make the diagnosis until they performed the actor’s autopsy.

“I know now the doctors, the whole team was doing exactly the right things,” Susan continued. “It’s just that this disease was faster than us and bigger than us. We would have gotten there eventually.”

Robin goes on sale May 15.