For many, Robin Williams' death by suicide came out of nowhere in 2014.

The iconic comedian, 63, had appeared in new films and talk shows that showcased his wit.

Reports around his death said that the suicide was a result of severe depression and due to the actor being diagnosed with a mild case of Parkinson's disease earlier in the year.

An autopsy proved this to be untrue.

His family learned that Williams had been suffering from a neurological disorder known as Lewy Body dementia that can cause hallucinations and dangerously acting out dreams while sleeping.

A new documentary called Robin's Wish documents his final days as his condition worsened and had his family and loved ones fearing for the worst.

“We had unknowingly been battling a deadly disease,” Williams’ widow Susan Schneider Williams, 56, says in the documentary.

“A disease for which there is no cure. The devastation on Robin’s brain from Lewy bodies was one of the worst cases medical professionals have ever seen, yet throughout all of this his heart remained strong.”

Family and friends confirm that Williams began to deteriorate two years before his death.

“I would say a month into the shoot (of Night at the Museum 3), it was clear to me — it was clear to all of us — that something was going on with Robin,” director Shawn Levy says in the doco.

“That’s an experience I’ve not spoken about publicly ever. We saw that Robin was struggling in a way that he hadn’t before to remember lines and to combine the right words with the performance.”

His neighbour John Hepper explained his concern over William's quickly changing body.

“His ribs were actually showing (through his T-shirt),” Hepper says in Robin’s Wish. “I grabbed his skin. ‘Robin, you’re really getting thin.’ He said, ‘Yeah, boss, I’ve gone to the doctor, but they don’t know what it is.’”

William's widow was surprised at how quickly the paranoia was brought on.

“The degree to which the paranoia came in was so drastic,” Susan says. “He’s going from room to room and literally watching me. He’s making a lot of phone calls and texting people and questioning … my loyalty to him.”

The night of William's death, his neighbour Hepper spotted him outside.

“Boss, I really need a hug,” he remembers Williams saying. “So, I gave him a hug, and he started to cry.” Hepper put his arm around the actor’s shoulder and spoke in depth with him for 15 minutes.

“He talked about family, and what was going on in his life and some things I think he felt that I would keep private,” he says.

The next morning, when Williams' assistant tried to enter his office and found the door locked, Susan knew he was gone.

Williams left no note.

Susan continues to cherish her husband's memory as a generous man and often reflects on what he would want his legacy to be.

“For Robin, it was that he wanted to help people be less afraid,” she says.