If Jackie Chan ask either of them who are willing to strip for a half-sex scene, Zhang Ziyi will "Kee Chiu" immediately while Sharon will take a few more minutes to ponder. As director, the right candidate is a no brainer. Sinkies actress are all "shy", which has to do with our conservative society. Playing safe and Kiasi is their best option, no spur in their behind as LHL put it.