It took Steven Spielberg's clout and an unusual promise to get pals Clint Eastwood and Mel Gibson to make cameos in Casper. ''The deal was that Steven [who is an exec producer] told them, 'I'm going to do a cameo. Will you?''' says Casper director Brad Silberling. Unfortunately, it was Silberling who had to inform his boss he was being left on the cutting-room floor. ''Our montage of cameos was getting long,'' he says. ''I had to tell Steven, 'You're not the strongest of the group.''' Spielberg took the edit in stride. ''Steven was sort of relieved,'' adds Silberling. ''He felt compelled to do the cameo since he asked for favors, but he's not an actor. Doing the cameo, he was as nervous as anything. He was shaking.''