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.