Brad Pitt is naked, sweaty, and the 15 kilograms of extra muscle he love scene with the actor often described as the sexiest man alive? "She is a beautiful, uncomplicated, simple, pure actress and a very nice girl," O'Toole.
Alex von Tunzelmann wonders why the makers of Troy bothered tackling guest etiquette, and sneaks off to have sex with Menelaus's wife, Helen. In classical history, Achilles was disguised as a girl to avoid going to war.
That's really up to the actor (if they have enough clout). Very often it is written into contracts whether an actor will go fully or partially nude.