In his memoir, Did I Ever Tell You This?, Neill recalled his experience working with Williams on Bicentennial Man (1999), and told how he developed a close friendships with the comedian during filming.

In the book, the New Zealand actor – who recently revealed he’s been contending with stage three cancer – said Williams was the “funniest” and “saddest” man he had ever met.

In excerpts of his memoir published by People, Neill wrote: “We would talk about this and that, sometimes even about the work we were about to do.”

He described Williams as “irresistibly, outrageously, irrepressibly, gigantically funny.”

Yet Neill also spoke candidly on the pain he saw beneath his co-star’s comedic facade.

Neill wrote: “He had fame, he was rich, people loved him, great kids—the world was his oyster. And yet I felt more sorry for him than I can express.

“He was the loneliest man on a lonely planet.”

What’s more, Neill added that Williams was “inconsolably solitary, and deeply depressed”, claiming he could sense a “dark space inside from the minute he flung open the door.”

That Williams was locked in a fight with powers determined to destroy his life cannot be questioned. In 2014 he was diagnosed with Parkinson’s disease, but this would later turn out to be a misdiagnosis – after his death it was discovered that he in fact had Lewy body dementia.

Susan Schneider Williams, who married Robin in 2011, said: “Nearly every region of his brain was under attack. He experienced himself disintegrating.”

She described his condition as “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.”