Who knew? Robert De Niro is a giggler. ''He gets into, like, conniption-fit giggles towards the end of the day when you only have four minutes to get the shot,'' reports Rebecca Romijn-Stamos (''X2''). Luckily, the horror thriller needed some comic relief: Romijn-Stamos and Greg Kinnear play parents recruited by a mysterious doctor (De Niro) aiming to clone their deceased 8-year-old son (Cameron Bright). But when the replacement child turns 8, major glitches begin. A highlight of the film, notes Kinnear, is the child's transformation from good to evil. ''I would be turned off by the movie if all of a sudden we were at a point where, 'Oh, now he's the bad kid,''' says the actor. ''You won't know as you're watching it where that moment is.'' Adding to the on-set creepiness, says Romijn-Stamos, were news headlines that eerily mimicked the script. ''It was at the time [the religious group] the Raelians claimed that they had [cloned someone]. So the movie started out as sci-fi and now it's just...sci.''