Robin Williams -- seen here with his movie son, actor Josh Hutcherson, in a photo from RV -- says having "kids who are as real as they come" gives the film a real spark.

NEW YORK -- You don't have to be stupid on screen to be funny, according to comedian Robin Williams.

All it takes is an infusion of reality to make even the physical, slapstick comedy work, Williams tells the Sun.

"Without (the reality), it's just slamming into things," he muses in a one-on-one interview that follows a group session set up to promote RV, his newest family comedy. It opens Friday. "If you start with that physical comedy, then things surely but slowly fall apart.

"But you build on that reality and then you can go pretty outrageous -- up to the Keaton-like gags."

With RV, the tale of a fractured family vacation, Williams does go to extremes, including in a priceless scene in which, like stone-faced, silent film master Buster Keaton, Williams defies gravity and the illusion of grievous injury to sell a stunt as his RV is perched on a cliff edge.

Despite trailers that make RV look like just a cheap, throwaway romp, there is a lot of heart in the story, too, especially in the family dynamics. Williams is the out-of-touch head of a family that starts off disconnected, but ends up united and loving.

"I think it takes Barry," Williams says of RV director Barry Sonnenfeld (Men In Black, The Addams Family, Get Shorty). At odds with Jim Carrey, Sonnenfeld abandoned the remake of Fun With Dick & Jane and then moved on to RV, where he had rapport with Williams.

Both knew the family vacation routine from personal experience, Williams says. "I think it takes the reality of knowing, of being honest with it, having been through it." That meant avoiding the Hollywood trap: "They make these outrageous comedies and just use these kids as props," Williams says. "They're not, they're beings who are transforming."

His own personal family vacations helped him make RV both funnier and more realistic, Williams says. Most of those trips were in the family's Land Rover from their home in San Francisco's Seacliff neighbourhood up to a rented house in the mountains around Lake Tahoe. Williams has a favourite trip, for its absurdity.

"On the way up there (once), we ran into this blizzard that was like Donner Party Time," Williams says with a manic grin. "You start looking at the kids going: 'Ummmmm, tasty! And your little fat friend, glad we brought him!' And we ended up staying in this motel just like the one in Psycho."

For the Williams clan -- Williams, his second wife Marsha Graces, their 16-year-old daughter Zelda and their 13-year-old son Cody -- these adventures were the same as those enjoyed by most folks. There is nothing celebrity or elitist about their activities.

"That's why I like going up to Tahoe," Williams says. The family members play miniature golf, jump in the frigid waters of the lake, scoot around on bicycles and act normally. Zelda smuggles in her iPod to listen to music, despite an edict that electronics are forbidden.

"After a while, you just kind of chill," Williams says. "It just becomes a whole different lifestyle and no one bothers you. They don't care (that he is a celebrity), not up there. They're vacationing too. They don't want to be bothered either. It's fun. I like that."

In RV, the movie plays the fictional family's vacation as both comedy and wholesome, realistic drama. "The moments of comedy are there," says Williams.

"But the whole idea of a vacation is the stripping away of stuff and then, by the end of the vacation, (at least) in my life, you do have these moments as a group, as a family having a great time, so mellow again. It takes a while to do that but that's what this movie is about."

Williams says his on-screen family, as well as the rival family headed by co-star Jeff Daniels, helped put across the heart in the movie. He pays tribute to Cheryl Hines, as his wife, and singer-actress Joanna (JoJo) Levesque and actor Josh Hutcherson as their kids.

"You have to have kids who are as real as they come, like JoJo and Josh. I love the territorial stuff: 'That's mine!' And they'll just fling each other across the room. The things that they do to each other! Anyone who has more than one child knows that that's what goes on. They almost did it to the point where you're going: 'They're going to kill each other!' "

It's real ... and it's funny.

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