Studios usually want as little input as possible from authors whose books they’re making into movies. But if Paramount gets the highly sought-after Leonardo DiCaprio to co-star with Richard Gere in “Primal Fear,” the studio will owe a debt of gratitude to William Diehl, who wrote the novel on which the movie is based. He also did the first script draft, with “Schindler’s List” scribe Steve Zaillian turning in a rewrite shortly for an April 17 shoot.

Diehl lives on the island of St. Simons, off Georgia, where he’s neighbors with an ex-cop named Art DiCaprio – who kept mentioning that when Diehl’s script was ready, he’d be happy to show it to his nephew, Leo. “I had no idea who he meant, because I’d never seen him in a movie at that point,” said Diehl. “Neither ‘This Boy’s Life’ nor ‘What’s Eating Gilbert Grape’ played in the theater here. We don’t even get Woody Allen movies.”

After producer Gary Lucchesi told Diehl that Gere agreed to play the lead role of district attorney who defends a homeless youth accused of brutally murdering a Catholic arch-bishop, Lucchesi mentioned they hoped to get DiCaprio for the youth. And suddenly, his pal’s offer clicked. After Art told Leo’s dad, George, Diehl got George a script. The offer’s now before DiCaprio, and Diehl, now a DiCaprio expert, thinks he’d be perfect.