Johnny Knoxville knows what it must sound like when he explains the plot of his next movie, "The Ringer."

"It's about a guy who gets hard up for dough, my uncle gets hard up with these bookies, and he convinces me to fake like I'm mentally challenged to get into the Special Olympics," he said. "On the surface it sounds like it's a mean-spirited movie, but it's not. It's a Farrelly brothers picture, so it's sweet."

And not just sweet in a "There's Something About Mary" or "Dumb & Dumber" sort of way, but like a funnier "Rain Man."

"We cast real mentally challenged actors in the roles, and it's one of the coolest things I've ever done because all the kids are brilliant," Knoxville said. "They're so cool because it's like something's taken away in one area, but in other areas they're just brilliant. They're like Rain Men. A lot of them call me every other day, and they're great."

The Farrelly brothers produced the movie, which Barry W. Blaustein, who wrote "Boomerang" and "The Nutty Professor," directed from a script by veteran TV scribe Ricky Blitt ("The Family Guy"). Katherine Heigl ("Roswell") plays Knoxville's love interest.

As politically incorrect as "The Ringer" might sound, Knoxville has already topped it with another upcoming movie, "A Dirty Shame," which finds him working with a kindred spirit, director John Waters ("Hairspray," "Pink Flamingos," "Polyester"), otherwise known as "The Sultan of Sleaze," "The Pope of Trash" and "The Baron of Bad Taste."

"It's one of his funniest and naughtiest pictures in years," Knoxville said. "I'm a huge Waters fan." The film stars Tracey Ullman as a Baltimore convenience-store clerk who suffers a concussion that causes uncontrollable sexual urges. Chris Isaak and Selma Blair co-star. Neither "The Ringer" nor "A Dirty Shame" have opening dates, but both are expected before the end of the year.

Meanwhile, Knoxville is preparing to join Heath Ledger, Emile Hirsch and Nikki Reed in "Lords of Dogtown," a skateboarding biopic based on the documentary "Dogtown and Z-Boys." Catherine Hardwicke, who earned rave reviews with "Thirteen," is directing (see "Fred Durst No Longer Directing Skateboarder, Mob Flicks").

Knoxville's latest movie, "Walking Tall," starring the Rock as the county sheriff of a corrupt small town, opens Friday. "He's a really cool guy and he's got a good sense of humor," Knoxville said of his co-star. "And there's no pretense about him. He just seems like a guy you grew up with, so we got along off-screen too."