NEW YORK -- Robin Williams knows the power of laughter.

It changed his life and he has observed how it affects others.

"Christopher Reeve and I went to Julliard together. When I learned of his accident I was as devastated as everyone else," recalls Williams, who rushed to Reeve's bedside.

"People were so solemn. I knew it was not good for Chris, so I dressed up in hospital scrubs and pretended to be his proctologist.

"The smile on his face almost broke my heart. He has told me since that it was at that moment when he was able to laugh again that he wanted to live."

For more than a decade, Williams has been visiting children's hospital wards in San Francisco where he lives.

"I usually go at Christmas. I ride a bike hooked up to an IV. I used to be a real hit when I did Mork (from the Mork and Mindy TV show) but now they love it most when I break into Mrs. Doubtfire."

It was for these reasons that Williams was so eager to play Hunter Patch Adams in the holiday movie Patch Adams. It's the true story of a physician who uses humour to treat seriously and terminally ill patients.

"His critics call him a modern Don Quixote who's deluded, but Patch is not chasing windmills," says Williams. "He's committed, dedicated and intelligent and he does everything possible to help his patients."

There is a particularly heartwarming sequence in Patch Adams when Williams entertains a ward of children coping with cancer.

"Most of the children in that scene really are cancer patients. They got their roles in the movie through the Make a Wish Foundation. Their reactions are spontaneous. It's not acting."

When Adams visited the set of the movie, Williams was anything but intimidated.

"We became instant friends. He made me laugh so much it hurt. He's an outrageous guy, an absolute born clown."

Williams is quick to point out that the film "is more about Patch's essence than it is about him. It's not a biography. Some of his life is in it, but some of it is fiction."

Williams, who turned 47 this year, was born in Chicago. When he was 16, he moved with his parents to San Francisco, the city he calls his only real home.

"In Chicago, I was a shy, lonely boy who excelled in sports. I knew my parents loved me, but I sensed they didn't know how to communicate that love.

"We moved to San Francisco because my father took early retirement. It was only then that he let down the wall that had built between us. It's one reason I love San Francisco. It's where I found my father."

The other reason is that Williams no longer felt like an outsider.

"No one is strange in San Francisco. It's a wonderful, eclectic place. Even now, I can ride my bicycle anywhere or even walk and I never get hassled."

It was in San Francisco that Williams found his funny bone.

"I started doing standup and people responded.

"I knew I wanted to be an entertainer, so I went to New York to study theatre at Julliard."

Immediately after graduation, Williams hit the American comedy circuit, where his wildman antics turned him into a star.

"For me, standup is akin to possession. I get up there and suddenly I turn from Dr. Jekyll to Mr. Hyde. It's still the way it works for me, which is why I prefer acting, especially at my age."

Williams lives in San Francisco with his second wife Marsha and their children Zelda, 9, and Cody, 7. Williams' son Zach, who is 15, lives part of the time with his mother Valerie Velardie and part of the time with his father.

"When you have three children, you can't be a wild man. They demand that you be grounded. I also stop myself from doing things which I think might embarrass them.

"There's nothing I could do that would embarrass me, but they are easily offended by some of my antics."

Williams has already completed work on the drama Jacob the Liar, set in the Holocaust, and will begin work early next year on The Bicentennial Man in which he will play a robot who wants to be human.

There is already a buzz that Williams is a front-runner for this year's Oscar nominations. He won last year for best supporting actor for Good Will Hunting.

"It's too early for me to win again. If I'm going to be honoured with multiple Oscars, I want to be like Jack Nicholson and have one Oscar for every decade."

Williams' Oscar sits on his wife's desk in her office.

"He's the paper weight for the mail. It's a way to remind us of our good fortune in this business."