The Jurassic Park actor opened up about his friendship with the late actor in his new memoir Did I Ever Tell You This?

Sam Neill is reflecting on his friendship with Robin Williams.

The actors worked alongside one another in 1999’s Bicentennial Man, which Neill, 75, reveals in his new memoir, Did I Ever Tell You This? was a time the two would have “great chats” throughout their visits to each other trailers.

“We would talk about this and that, sometimes even about the work we were about to do,” explains Neill, adding that Williams ” was irresistibly, outrageously, irrepressibly, gigantically funny.”

While Neill credits the late actor as the “funniest person” he ever worked with, he shares that Williams was also “the saddest person I ever met.”

“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,” writes the Jurassic Park star, expressing that he remembers Williams seeming “inconsolably solitary, and deeply depressed.”

Robin Williams. ETHAN MILLER/GETTY IMAGES

While Neill shares that “he could sense the dark space inside,” during his conversations with Williams, “as soon as he flung open the door, he was on.”

“Funny stuff just poured out of him. And everybody was in stitches, and when everybody was in stitches, you could see Robin was happy,” explained Williams.

Williams died by suicide at the age of 63 on Aug. 11, 2014. It was later discovered the actor had Lewy body dementia, the second-most common type of progressive dementia after Alzheimer’s disease.

After his death, a representative for Williams shared with PEOPLE that “he had been battling severe depression of late.”

In 2020, his son, Zak Williams, detailed his father’s experiences with mental health on an episode of The Dr. Oz Show

“I was acutely aware of my dad’s struggles with depression, it manifested in addiction at times, and he took great lengths to support his well-being and mental health, especially when he was challenged,” he shared. “It was something that was a daily consideration for him.”

He continued: “The main thing for me was noticing how he went to great lengths to support himself while he could show up for others. It was clear that he prioritized his mental health throughout most of his life, at least that I experienced with him.”

If you or someone you know is considering suicide, please contact the 988 Suicide and Crisis Lifeline by dialing 988, text “STRENGTH” to the Crisis Text Line at 741741 or go to 988lifeline.org.