Fellini had his Giulietta. Hitchcock had his cameos. And Mel Gibson has his...butt. From Gallipoli and Lethal Weapon to Bird on a Wire and Forever Young, Gibson's derriere has been flashed on screen about as often as his baby blues. It's on display once again in Braveheart, the epic story of 13th-century Scottish rebel William Wallace. But this time the actor-turned-director insists there's a historical precedent to a cheeky scene in which the Scots moon their English counterparts and then turn around and flip their kilts again right before the battle begins. ''The Scots used to really do that,'' Gibson says of the double exposure, adding, ''I did it as an answer to the question, 'What do Scots wear under their kilts?''' Alas, Gibson filmed the scene at such a safe distance that much of Mel is still left to the imagination.