On the dark side - Director George Ratliff and Jacob Kogan on the set of Joshua.

WHEN GEORGE RATLIFF'S WRITING PARTNER, David Gilbert, told him his idea for a suspense thriller about a sinister child prodigy, he wanted nothing to do with it.

"I was reluctant, because the horror genre is not that interesting to me," Ratliff says in a phone interview. "I love the horror movies of the '70s, but I don't like the supernatural." Besides, Ratliff and his wife had just had their first child, and he was uncomfortable with the theme. "I didn't want to do this story about a bad kid. I was just trying to raise my son."

Ultimately, Ratliff couldn't resist the lure of the project. He agreed to help write the script and direct the movie, Joshua, which opens here Friday. Joshua is a glossy, sophisticated urban gothic about an affluent New York family: Wall Street trader Brad (Sam Rockwell), his wife Abby (Vera Famiga) and their 9-year-old son, Joshua (Jacob Kogan), a quiet, well-mannered piano prodigy. Strange things begin to happen in the family's elegant Manhattan apartment after they bring their newborn daughter home. Creepy noises emanate from upstairs. Pets die mysteriously, then family members as well. Abby, who is prone to postpartum depression, begins to unravel. Joshua, who has mastered a difficult Beethoven sonata for a school recital, launches instead into a dissonant version of "Twinkle, Twinkle Little Star," then collapses onstage. What is going on?

Strangely (and maddeningly), the screenplay never really answers this question. The film offers up a deli case of red herrings - suggestions of ghosts, implications of demonic possession, a crazy Christian fundamentalist grandmother, Joshua's obsession with ancient Egyptian burial rites. But in the end, it's unclear whether Joshua is moved by supernatural forces or simply a desire to rearrange the family dynamics in his favor.

Ratliff, 39, explains that he and Gilbert wanted to avoid the kind of metaphysical rationales offered in movies like The Omen, Rosemary's Baby and The Demon Seed. "There's an [audience] expectation that there'll be some kind of explanation," he says. "But it's much scarier that there's no good reason that [Joshua] is the way he is." Rather than externalizing the evil onto ghosts or demons, the movie touches on a primal sense that children are always alien to their parents - especially a child like Joshua, whose musical genius makes him so different from his family. "He's sort of unknowable to them," says Ratliff. "He goes through life being the perfect child, playing this correct music. Then he finds he has a power that is so much more interesting. He finds there's a lot of power in playing the wrong notes."

Ratliff first earned attention as a director of documentaries. After a brief foray into newspaper journalism in Texas and Costa Rica - a route he took only to persuade the University of Texas film program to accept him - he directed a couple of well-received documentaries set in his home state of Texas: Plutonium Circus, about an atomic bomb factory near Amarillo, and Hell House, about a Halloween funhouse staged by a Dallas fundamentalist church. Joshua is his first major fictional film, and he enjoyed the freedom of the process. "With the documentaries, I had to figure out every scene on the fly. So for me, this was a bit of a luxury. I could storyboard and rehearse the actors." He and Gilbert are currently working on an adaptation of Don DeLillo's novel End Zone, a football-themed farce.

Reviews of Joshua have been mixed, but some, including The New York Times, have praised its acknowledgement of the seldom-mentioned dark side of parenting. In one scene, Joshua taunts his dad in the park, and the beleaguered Brad erupts in a rage and beats him. Ratliff says audience members seem to enjoy seeing Joshua get his comeuppance. "I think our instincts are to give a child the benefit of the doubt," he says. "But people were mad at the kid. I think they felt he deserved it."