Billy Crystal thinks his friend Robin Williams' death still feels ''like a second ago''.

The 70-year-old actor was devastated when his pal took his own life almost four years ago but thinks new documentary 'Robin Williams: Come Inside My Mind' is a fitting tribute to the 'Mrs. Doubtfire' star.

He said: ''It's gonna be four years in August and it just-- it feels like a second ago, but I just hope that people will come and see this movie and understand a little bit more about the man and what made him this explosively funny genius.''

Billy thinks the documentary will allow fans the chance to get to know Robin better as it shows his more private side.

He told 'Entertainment Tonight': ''Well, you know, when they see somebody in a movie, or in a concert or in six minutes on a talk show or whatever, that's what they get. And this film gets you behind that and lets you get to understand how that happened. You know, how that kind of mind was created.

''[He was a] lonely kid, a kid on his own a lot, inventing characters, inventing whole worlds and as he grew, it got more complete and easier for him to get to.''

The 'City Slickers' star found it a ''privilege'' to ''trade blows'' in interviews with his friend.

She said: ''He always looked to me to bring him back to earth and I could take him in a different direction and it took me a while to figure out how to counter-punch and score my own points, you know... it was a privilege to trade blows with him.''

But Billy didn't want to talk about Robin's private battle with depression as it would be a betrayal of their friendship.

He simply said: ''You know, that's really stuff I don't talk about. Incredibly difficult times.

''You know, friends trusting each other with the darkest of secrets that I'll keep that way.''