BILLY Bob Thornton might seem an odd choice to play Santa Claus – but this isn’t just any Santa Claus.

In the hilarious new movie “Bad Santa,” which opens next Wednesday, Thornton plays a drinking, smoking, thoroughly corrupt department store Kris Kringle.

He not only steals from the stores where he works, he preys on women in the plus-size department.

“I’m an eating, drinking, f – – – ing Santa Claus!” Thornton boasts to a bartender played by Lauren Graham.

“Prove it!” she answers – and soon the two are going at it in Thornton’s car.

The movie is so depraved it would make the Farrelly brothers blush.

“It’s the dark side of ‘It’s a Wonderful Life,’ ” Thornton told The Post.

That troubled some executives at Disney, which owns Miramax, the film’s distributor.

On Sunday, the Drudge Report quoted an unnamed “top source” at Disney as saying the film “is just not in the spirit of Walt Disney.”

Miramax says such rumors have been blown out of proportion.

“No one at Disney ever expressed their concern about the film,” says Miramax spokesman Matthew Hiltsik. “People need to get a sense of humor.”

Director Terry Zwigoff insists Thornton’s character really isn’t that bad, since he befriends a geeky child and helps him gain self-confidence.

“It’s like one of the characters that Walter Matthau used to play – this gruff, grumpy, unlikable old cuss that you grow fond of in spite of yourself,” says Zwigoff, who took over the movie after it was developed by the Coen brothers.

Another surprise about “Bad Santa”: It includes John Ritter’s last film role.

The late actor plays an uptight department store manager, who suspects that Thornton’s Santa is up to no good, but can’t fire him because he works with an elf (played by Farrelly favorite Tony Cox) who’s both African-American and a midget.

Ritter based his portrayal on a character he used to play for laughs when he was goofing around with Thornton.

Thornton and Ritter have been good friends since working together on the early ’90s sitcom “Hearts Afire” and in Thornton’s breakout 1996 film, “Sling Blade.”

“He had this guy he did called Chuck Hurley, who was on the verge of vomiting all the time,” Thornton recalls.

“Chuck would tell these stories about how he couldn’t get any women to kiss him, and the whole time, he’d look like he was just about to puke.

“People always think of John as the guy next door,” Thornton says, “but he had a really twisted sense of humor, and we connected on that.”

“Bad Santa” also stars Bernie Mac, as the corrupt manager of store security, who tries to blackmail Thornton and Cox for part of their stolen loot and is prone to exclamations like “Sweet Jews for Jesus!”

Zwigoff says had trouble finding someone to play the geeky kid, whose name is Thurman Merman.

“I wanted some kind of cretinous mongolid for the part, and they kept bringing me these cute Hollywood kids,” Zwigoff says.

Casting agents found a tubby Canadian grade-schooler named Brett Kelly just three days before filming was to start – and he bravely put up with scenes like the one where he gets in the boxing ring for a midget-vs.-kid fight with Cox.

But Thornton really propels the movie, and despite his reputation as Hollywood’s weirdest actor – what with Angelina Jolie’s vial of blood and that all-orange-food diet – he says he wasn’t always a professional oddball.

“I was one of those kids who believed in Santa longer than I should have,” says the 48-year-old Oscar winner, who will spend both Thanksgiving and Christmas at home in Beverly Hills with his sons Willie, 10, and Harry, 9, and their mother, his ex-wife Pietra Dawn Cherniak, a former Playboy Playmate of the Year.

“I wanted so badly to believe in it.”

– with Post wire services