Simply put: players decide to actually role play. Either by using reference to mechanics (as in Pendragon with it's trait and passion system), or by doing a bit of study (as in historical Japan as a setting, or Europe in Ars Magica).
For many players, it's as easy as giving them a list of significant role elements for the setting. Players actually interested in historical settings also tend to do some research.
It's no different than trying to play one of the aliens from the Classic Traveller Alien Modules... the key points are elucidated, and the group interpolates from there. It may not be 100% right to the authorial intent, but it is a challenge to do, and some players thrive on that challenge.
Historical RP may not be 100% accurate, but it can create compelling stories that a modern politically corrrect setting doesn't. It can create challenges that force players outside of the modern mindset, and allows them to understand history better.