Although it’s fairly obvious that people should behave appropriately in public spaces, it’s important to remember a few things when using the Paris Metro and RER as a tourist.
Firstly, bear in mind that it is a commuter system and people are often in a rush to get to work, home, etc. Try not to stand in the way or block up doorways, entrances, and exits, and if you aren’t sure where you’re going, move to the side – Parisians can be impatient (understandably so) and you don’t want to experience antagonism on your holiday.
Secondly, for your own safety and others’, it is important to always be alert and to follow the rules carefully.
RATP has created a nifty little etiquette guide for using the metro, which you can view here: Manuel de savoir-vivre.