In Robin Williams’ only dramatic Broadway performance, “Bengal Tiger at the Baghdad Zoo,” the late actor was said to have waltzed seamlessly through his roughly 500 lines without a glitch.

Barely two years later, reprising his role as Teddy Roosevelt in “Night at the Museum: Secret Tomb,” Williams battled to remember one line of his cameo role.

One line. And neither Williams, his wife, Susan Schneider Williams, and baffled doctors could pinpoint the exact cause of the swift deterioration of the comic’s mind.

It wasn’t until an autopsy following Williams’ Aug. 11, 2014 suicide revealed that the performer suffered from perhaps the worst known case of Lewy Body Dementia — a disease a level beyond Parkinson’s that derails the brain’s neurons.

Educating the public — and crusading for a treatment, if not a cure — of LBD became Schneider William’s unwanted destiny.

“Robin’s Wish,” a 77-minute documentary released Sept. 1 on Amazon Prime, takes a major step in explaining the disease, ending inaccurate speculation, and perpetuating the genius of Robin Williams.

Ultimately, Schneider Williams said she hopes the film “sets the record straight on Robin’s legacy. I wanted people to get the closure that they needed and deserved. When I found out what had happened (the LBD diagnosis), that’s when I began to heal. I was painfully aware of the misinformation out there. I wanted justice for Robin and everyone who didn’t have the correct information so they could have healing and closure and for the people who also suffered from this brain diseases, for them to know they’re not alone. That was a huge part of it.”

The documentary offers clips of vintage Williams, interviews with fellow Bay Area stand-up comics and film directors, and a plunge into the dark waters of of brain disease few heard of — until Williams’ diagnosis.

“While Robin’s case was unique, we’re not alone. There are many who do not have a diagnosis,” Schneider Williams said from her Marin County home. “There is no definitive diagnosis (on a living person). It can take a couple of years or a zillion years to get to the bottom of it and that’s not right.”

Schneider Williams witnessed her husband’s memory loss, the sleep deprivation, delusions, and general cognitive impairment.

“The last nine months .. the progression was rapid,” said Schneider Williams, describing her husband’s brain synapses as “a Jackson Pollock painting.”

“The thermostat to regulate everything that makes us human was breaking,” she said.

Even with zero treatment or cure, knowing her husband had LBD while he was alive would have been easier to take and beneficial, Schneider Williams said.

“It’s the most common disease you’ve never heard of,” she said. “There is power in diagnosis. There would have been a language to talk about. Robin could have been part of a clinical trial.”

Instead of endless “chasing down the rabbit hole, maybe Robin would have rather be sitting on a beach. Because of everything I’ve learned about the disease, he would have known he was not alone and suffering,” Schneider Williams said.

“Robin’s Wish” details how LBD put a death grip on her husband.

“It’s a tough thing to explain and understand,” Schneider Williams said.

“Robin’s Wish” only happened by the urging of Schneider Williams’ art framer, Jim Ringseis in Fairfax, who had admired a 20-minute 2014 documentary, “Naples Yellow” by director Tylor Norwood.

“Jim kept saying, ‘I think you should reach out to the director.’ That was the last thing I wanted to do,” said Schneider Williams.

Meanwhile, she became involved in the American Brain Foundation, joining the board of directors five years ago.

“I was following the science of what Robin went through and Jim kept telling me to reach out to Tylor,” said Schneider Williams.

She surrendered, contacted Norwood, the two met … and “Robin’s Wish” happened. While Schneider Williams “felt it was safer for me to go through the science trail,” Norwood tracked down former neighbors, friends, and performers.

“I was proud of everyone who sat down and gave up their time and themselves. It was not easy to do that for the film,” Schneider Williams said.

That included two comics, Berkeley’s Johnny Steele and Rick Overton of Los Angeles, the rare performers who could keep pace — or stay within a joke’s length — of doing improv with Williams.

Steele often accompanied Williams on bike rides, gaining intimate insight as few could.

“He and Susan really seemed to be madly in love. I think he really felt that she was the one, that he’d gotten rid of his demons, that he’d found some peace,” Steele said. “They were all over each other whenever I’d visit, whenever we went out for a ride.”

Hanging out with Williams convinced Steele that relocating to Los Angeles may have been a better career choice, but remaining in the Bay Area was the better life move.

“Robin making this his home made it clear that this was the place where the quality of life was much better and that a better quality of life was what really mattered,” said Steele, who has yet to see “Robin’s Wish.”

Overton said watching the fallout from the dementia on his longtime friend “was not an easy thing to see, but I’m 110 percent here for the film. Robin was killed by a stranger using his own hands.”

Though Schneider Williams had seen the documentary during the editing process, she actually paid for it a few days ago to see it on Amazon Prime.

“I saw it on my couch with my dog, Leonard, and cat, Spotty,” she said, laughing that the film “is worth every penny. It’s a good investment in your future.”

And watching herself be interviewed?

“It’s awful,” she said laughing. “There are a lot of other things I’d rather be doing.”

Right now, it’s continuing her art, promoting the documentary, and continuing her fight to solve Lewy Body Dementia.

“This is something I’ll do for the rest of my life,” Schneider Williams said.

An accomplished artist, Schneider Williams said that “for awhile” she wrote Robin cards, “sealed them and tucked them away” every year on his July 21 birthday.

On the annual day he died, “I paint him a panting. That’s how I spend the day. I feel like I’m with him,” she said. “And I probably talk to him a little more than usual.”

“Robin’s Wish” is available on Amazon Prime, iTunes, Google Play, and Vudu.