Scarlett Johansson, one of Hollywood’s leading sex sirens, is about to undergo an on-screen transformation that will shock her fans. She’s set to play a woman who masquerades as a man to run a string of massage parlors in the upcoming film “Rub & Tug.” It won’t be the first time an actress plays a man in a movie. But it should be interesting, nonetheless, because Johansson has always been associated with sexy roles.