AceShowbiz - Robin Williams' son Zachary has reflected on how tough it was to see his father suffer "intense personal pain" in an interview ahead of the fifth anniversary of the actor's death.

The "Mrs. Doubtfire" star was just 63 when he took his own life in August 2014. Zachary - whose mother is Robin's first wife Valerie Velardi - opened up about his late dad during an emotional interview with U.K. TV show "Good Morning Britain" last week (end July 21), as he spoke about his father's mental health struggles.

"It was sad to see someone who was suffering so... As a family member and a child, you want to do everything you can to help soothe and ease what seemed to be intense personal pain," he explained.

"And there were times where it felt like there was helplessness from my part, I didn't know what I could do, or how I could be of the best support. We all loved him so and found it difficult because he wasn't always open to sharing his personal pain and struggle... And we noticed that over a period of time."

Zachary added that he found the most "heartbreaking" thing was to watch his dad acting so jovially and making other people laugh when he was in such a dark place himself.

"He still went out and wanted to share his feelings of laughter and humour with the world," he mused. "While he was suffering and struggling, he still went out and performed. I admire him and loved him so, and having to share him (with the world) was hard."

As well as Zachary, Robin was father to two other children from his marriage to Marsha Garces - Cody and Zelda Williams.