Robin Williams ''didn't always feel'' like a success, according to his son.

The late actor - who tragically took his own life aged 63 in August 2014 - suffered from severe depression before his death, and his son Zak, 35, has opened up about the doubts his father had about his own career.

As seen in the first trailer for upcoming HBO documentary 'Robin Williams: Come Inside My Mind', he said: ''My father didn't always feel he was succeeding, but he was the most successful person I know.''

The biopic will feature never-before-seen footage of the comedian and actor - who suffered from Lewy Body Dementia towards the end of his life - alongside candid conversations with friends and family, including fellow stars like Eric Idle, Whoopi Goldberg and Steve Martin.

With director Marina Zenovich at the helm, it look to give fans' an insight into Williams' life, as he admitted to suffering from a childhood fear of abandonment.

In previously recorded interview, he said: ''Yeah, it's a primal fear for any child and it dictates a lot of how you deal with life.''

He's also seen suggesting that everyone is motivated by a ''deep secret'', and his made him them either face their fears or ''run away from it all''.

He said: ''Every person is driven by some deep secret. There was fear sometimes to run away from it all, sometimes to run into it all.''

Meanwhile, Zak previously opened up about how much he missed his dad ''all the time'', and thought of him when he spotted things his father would have enjoyed.

Speaking just four months after the 'Mrs Doubtfire' star's death, he said: ''I miss him all the time. Often I see something or if I'm watching a film, I think, 'Oh, man, he would have appreciated this' or 'He would have gotten a laugh out of this.' ''