Robin Williams says he is a

Actor Robin Williams, star of the film "World's Greatest Dad", poses at the film's premiere in Los Angeles, California August 13, 2009. REUTERS/Fred Prouser

Robin Williams stars in a movie called "World's Greatest Dad," but as a real life father of three grown kids the Oscar winner said he had to overcome dark times and was still a work-in-progress.

"I'm so proud of them in different ways, but have they always been cherubs? No, but that's been part of the process," Williams said in an interview.

"And am I the 'world's greatest dad?' Not at all, I'm a work-in-progress, but I love them," said Williams, whose latest of 51 movies opens on Friday.

A son from his first marriage, Zachary, 26, graduated from New York University. From his second marriage were born Zelda, 20, a budding actress, and Cody, who turns 18 this year.

The comic, who in the past has entered rehab to deal with substance abuse, said his difficulties gave his children a ready response when he questioned them about their behavior.

"They went, 'And you had a three-year drunken relapse.' Ah, thank you for bringing that back, my little happy creatures," Williams quipped.

After Zachary graduated from New York University with a degree in rhetoric, Williams joked that his son was qualified to "open a syntax repair shop."

But Williams said he sees how Zachary has pursued his own interests, just as Zelda has made her mark in horror movies and Cody was "kind but brilliant."

Zelda, has starred in several slasher-style flicks but Williams has begged off seeing them to avoid watching her get "killed" on screen.

In "World's Greatest Dad," directed by fellow comedian Bobcat Goldthwait, Williams plays Lance, a poetry teacher whose teenage son attends the school where he works.

The son Kyle (Daryl Sabara) is a rude outcast, but when he dies suddenly, Lance sets out to rehabilitate Kyle's posthumous image by "ghostwriting" for his son in a journal that circulates through the school.

Soon, Lance finds the disgust other students once had for Kyle transforms to idolatry.

"The movie is a bit of a satire about how we rewrite people when they die," Goldthwait said in the same interview.

"Now, suddenly everybody would let Michael Jackson baby-sit their kids," he joked, to illustrate his point.

Goldthwait and Williams are long-time friends, having performed together several times in comedy clubs under the pseudonyms "Jack Cheese" and "Marty Fromage."

After daughter Zelda saw this latest movie, she complained about not being warned of one scene in which Williams bares his private parts.

"Well you know," said Williams in a Shakespearean voice, "there are some things father's don't talk about."