Amblin/Universal’s big-budget specter spectacle “Casper” has further fleshed out its starring cast, with 12-year-old commercial thesp Malachi Pearson set to do the voice of the friendly ghost.Joe Nipoti (“Viper”), standup comic Brad Garrett and newcomer Joe Alaskey will star as Stretch, Stinky and Fatso, the ghostly trio who haunt Casper. They join Bill Pullman, Christina Ricci and Cathy Moriarty. The filmmakers will use a mix of live action and high-tech three-dimensional computer animation, similar to that used to create the dinosaurs in “Jurassic Park.” The film, which started production last week, is directed by Brad Silberling, a TV director (“L.A. Law,””NYPD Blue”) who is making his feature directing debut. Colin Wilson produces.